What Is a Pathogen?

A pathogen is a biological agent that can cause disease in a:

host.

Common pathogens include:

• bacteria
• viruses
• fungi
• protozoa

Some courses also include certain parasitic worms and other infectious agents when discussing pathogens.

The important idea is:

A pathogen can cause disease.

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What Is a Host?

A host is an organism in or on which a pathogen lives and may reproduce.

Hosts can include:

• humans
• other animals
• plants
• fungi
• microorganisms

For example, a virus that infects a human uses the human's cells and body as its:

host.

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Pathogen and Host

The relationship can be represented simply as:

Pathogen → enters host → survives or reproduces → disrupts normal functions → may cause disease

The pathogen benefits from resources provided by the host.

The host may be:

harmed.

However, infection does not always result in obvious disease.

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Infection and Disease Are Not Exactly the Same

An infection occurs when a pathogen enters a host and establishes itself or reproduces.

A disease occurs when normal body functions are disrupted and signs or symptoms develop.

Therefore:

infection does not always mean disease.

A person may sometimes carry a pathogen without showing symptoms.

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Are All Microorganisms Pathogens?

No.

This is an extremely important distinction.

Many microorganisms are:

harmless or beneficial.

Only microorganisms capable of causing disease under particular conditions are considered:

pathogenic.

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Microorganisms

A microorganism is an organism or biological entity too small to be seen clearly without magnification.

Examples include many:

• bacteria
• protozoa
• microscopic fungi
• microscopic algae

Viruses are also commonly discussed alongside microorganisms, although they are generally not classified as living organisms.

Some microorganisms cause disease.

Many do:

not.

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Non-Pathogenic Microorganisms

A non-pathogenic microorganism does not normally cause disease.

Many are important for:

• digestion
• decomposition
• nutrient cycling
• food production
• biotechnology
• maintaining healthy microbial communities

For example, some bacteria in the human digestive system contribute to normal:

gut function.

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Beneficial Bacteria

The human body contains enormous communities of microorganisms called the:

microbiota.

Many bacteria living in and on the body are harmless or beneficial.

Some help:

• compete with harmful microorganisms
• break down substances in food
• produce useful compounds
• support normal immune-system development and function

Therefore:

bacterium does not mean pathogen.

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Pathogenic vs Non-Pathogenic

Pathogenic microorganism

Can cause disease.

Example:

Some species or strains of Salmonella can cause gastrointestinal disease.

Non-pathogenic microorganism

Does not normally cause disease.

Example:

Many bacteria naturally living in the human gut are harmless or beneficial.

The key difference is their ability to:

cause disease.

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Major Categories of Pathogens

Four major categories commonly studied in biology are:

1. Bacteria

2. Viruses

3. Fungi

4. Protozoa

These pathogens differ greatly in:

• structure
• size
• reproduction
• transmission
• treatment

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Bacterial Pathogens

Bacteria are single-celled:

prokaryotic organisms.

Most bacteria are not harmful.

However, some bacteria can cause disease by:

• invading tissues
• damaging cells
• producing toxins
• triggering harmful inflammatory responses

Examples of bacterial diseases include:

• tuberculosis
• cholera
• some forms of food poisoning
• some forms of pneumonia

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Bacterial Structure

Bacterial cells typically contain:

• cell membrane
• cytoplasm
• ribosomes
• circular DNA
• cell wall

Some bacteria also possess:

• flagella
• capsules
• plasmids

Bacteria do not have a:

nucleus.

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How Bacteria Cause Disease

Different pathogenic bacteria use different mechanisms.

Some reproduce inside the host and:

damage tissues.

Others release substances called:

toxins.

These toxins can interfere with normal cellular or organ functions.

The immune response to an infection can also contribute to symptoms such as:

fever and inflammation.

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Viruses

A virus is much smaller and structurally simpler than a typical cell.

Viruses generally consist of:

• genetic material, either DNA or RNA
• a protein coat called a capsid

Some viruses also have an outer:

lipid envelope.

Viruses cannot reproduce independently.

They must infect:

host cells.

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How Viruses Cause Disease

Viruses enter host cells and use the cells' machinery to make:

new virus particles.

A simplified process is:

virus attaches → enters or delivers genetic material → viral components are produced → new viruses assemble → viruses spread

This can damage or destroy infected cells.

The body's immune response can also contribute to:

symptoms.

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Viruses Depend on Hosts

Viruses are completely dependent on host cells for:

replication.

Outside a suitable host cell, a virus cannot independently:

• reproduce
• carry out cellular metabolism
• grow like a cell

This makes the virus-host relationship particularly important.

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Viral Diseases

Examples of diseases caused by viruses include:

• influenza
• measles
• COVID-19
• chickenpox
• dengue
• HIV/AIDS

Different viruses infect different types of:

host cells and tissues.

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Antibiotics and Viruses

Antibiotics do not treat viral infections.

Antibiotics target structures or processes associated with:

bacteria.

Viruses have very different structures and replication processes.

Some viral diseases can instead be treated with specific:

antiviral medicines.

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Fungal Pathogens

Fungi are eukaryotic organisms.

They include:

• yeasts
• moulds
• mushrooms

Most fungi do not cause human disease.

However, some fungi can infect:

• skin
• nails
• lungs
• other tissues

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Fungal Diseases

Examples include:

• athlete's foot
• ringworm
• candidiasis

Fungi may grow on or inside host tissues and obtain:

nutrients.

Some fungal infections remain near the body's surface, while others can become more serious, particularly in people with weakened immune defenses.

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Protozoan Pathogens

Protozoa are mostly single-celled eukaryotic organisms.

Some protozoa are free-living and harmless.

Others are:

parasites.

A parasitic protozoan obtains resources from its host and can cause disease.

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Malaria

One important example of a protozoan disease is:

malaria.

Malaria is caused by parasites belonging to the genus:

Plasmodium.

The parasites are transmitted to humans through bites from infected female:

Anopheles mosquitoes.

The mosquito acts as a:

vector.

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What Is a Vector?

A vector is an organism that transmits a pathogen from one host to another.

Examples can include:

• mosquitoes
• ticks
• fleas

The vector and pathogen are not the same thing.

For example:

Plasmodium = pathogen

mosquito = vector

This distinction is important.

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Parasitic Worms

Some classification systems also discuss parasitic worms, or:

helminths,

alongside other pathogens.

Examples include:

• tapeworms
• roundworms
• flukes

Unlike bacteria or protozoa, many parasitic worms are multicellular and may be large enough to see without a microscope during some life stages.

They cause disease by living in or feeding from their:

hosts.

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Comparing Major Pathogens

Bacteria

Usually:

single-celled prokaryotes

Can reproduce independently under suitable conditions.

Some produce toxins or damage tissues.

Viruses

Non-cellular infectious agents

Require host cells to reproduce.

Fungi

Eukaryotic organisms

May be unicellular or multicellular.

Some grow on or within host tissues.

Protozoa

Usually:

single-celled eukaryotes

Some species are parasitic and cause disease.

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Size Differences

Pathogens vary enormously in size.

In general:

viruses < bacteria < many eukaryotic parasites

However, there is considerable variation within each group.

Size alone should therefore not be used to determine whether something is:

pathogenic.

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How Do Pathogens Enter the Body?

Pathogens can enter hosts through several routes.

These include:

• respiratory tract
• digestive tract
• broken skin
• mucous membranes
• sexual contact
• blood
• bites from vectors

The route depends on the particular:

pathogen.

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Respiratory Transmission

Some pathogens spread through respiratory particles released when infected people:

• cough
• sneeze
• talk
• breathe

Another person may inhale infectious particles.

Examples of pathogens that can spread through respiratory routes include those causing influenza, COVID-19, tuberculosis, and measles.

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Food and Water

Some pathogens spread through contaminated:

food or water.

They enter through the digestive system.

For example, certain bacterial pathogens can cause gastrointestinal disease after contaminated food is:

consumed.

Clean water, sanitation, and food hygiene can therefore reduce transmission.

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Direct Contact

Some pathogens spread through:

direct physical contact.

Others can spread indirectly through contaminated objects or surfaces, depending on how well the pathogen survives outside the host and the route required for infection.

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Blood and Body Fluids

Some pathogens can spread through particular:

body fluids.

Transmission routes may include:

• blood exposure
• sexual contact
• shared contaminated needles
• transmission from parent to child

Different pathogens have different transmission routes.

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Vector Transmission

Some pathogens depend on another organism for transmission.

For example:

infected mosquito → human host

The mosquito carries the pathogen between hosts.

Other vectors include some:

ticks and fleas.

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Colonization

Simply reaching the body is not always enough to cause infection.

A pathogen may need to:

colonize.

Colonization means establishing itself in or on the host.

The pathogen may need to:

• attach to cells
• obtain nutrients
• avoid removal
• reproduce

The body's defenses make this difficult for many microorganisms.

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The Body Has Defenses

Humans have several defenses against pathogens.

These include:

• skin
• mucus
• cilia
• stomach acid
• antimicrobial substances
• immune cells
• antibodies

These defenses help prevent pathogens from successfully establishing an:

infection.

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How Pathogens Cause Disease

Different pathogens cause disease in different ways.

Common mechanisms include:

1. Damaging host cells

2. Destroying cells during reproduction

3. Producing toxins

4. Taking nutrients from the host

5. Triggering damaging immune or inflammatory responses

Disease often results from a combination of these effects.

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Cell Damage

Some pathogens invade tissues and directly damage:

cells.

Viruses may damage cells while replicating.

Some bacteria invade tissues and interfere with normal cellular functions.

When enough cells are affected, the function of a tissue or organ may become:

impaired.

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Toxins

Some bacteria produce:

toxins.

A toxin is a harmful substance that can interfere with normal biological processes.

Even relatively small quantities of certain toxins can cause significant:

symptoms.

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Competition for Resources

Some parasites obtain nutrients directly from their:

host.

This can reduce the resources available to the host.

In severe infections, this may contribute to:

• weakness
• nutritional deficiencies
• tissue damage
• organ dysfunction

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The Immune Response

Some disease symptoms are caused partly by the body's attempt to:

fight the pathogen.

For example:

• fever
• inflammation
• swelling
• mucus production

These responses can help fight infection, but they may also make a person feel:

ill.

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Exposure Does Not Always Mean Infection

A person can be exposed to a pathogen without becoming:

infected.

The pathogen may fail to:

• enter the body successfully
• survive
• reproduce
• overcome host defenses

Therefore:

exposure ≠ infection

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Infection Does Not Always Mean Disease

A person can become infected without developing noticeable:

symptoms.

This is sometimes called an:

asymptomatic infection.

Therefore:

exposure → possible infection → possible disease

Disease is not guaranteed at every stage.

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Pathogenicity

Pathogenicity refers to the ability of an organism or infectious agent to:

cause disease.

Some microorganisms have a high capacity to cause disease in susceptible hosts.

Others rarely cause disease under normal circumstances.

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Opportunistic Pathogens

Some microorganisms usually cause little or no harm but can cause disease under certain:

conditions.

These are called:

opportunistic pathogens.

They may cause disease when:

• immune defenses are weakened
• they enter an unusual part of the body
• normal microbial communities are disrupted

This shows that pathogenicity can depend on both the microorganism and the:

host.

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Host Susceptibility

Not every host responds to a pathogen in the same way.

Susceptibility can be affected by factors such as:

• previous immunity
• vaccination
• age
• genetics
• nutritional status
• immune function
• amount of pathogen encountered

Therefore, disease depends partly on characteristics of the:

host.

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Pathogen Specificity

Some pathogens infect many different species.

Others have a narrow:

host range.

Even within one host, pathogens may target particular cells or tissues.

For example, some viruses infect mainly:

respiratory cells,

while others preferentially infect different cell types.

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Host-Pathogen Relationship

Disease is not simply a property of the pathogen alone.

It results from an interaction between:

pathogen + host + environment.

Whether disease develops can depend on:

• characteristics of the pathogen
• host defenses
• route of entry
• dose of the pathogen
• environmental conditions

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Pathogen Adaptations

Successful pathogens may possess adaptations that help them:

• enter hosts
• attach to host cells
• obtain nutrients
• reproduce
• avoid immune defenses
• move between hosts

These adaptations increase their ability to survive and:

spread.

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Transmission Is Important to Pathogens

For a pathogen population to persist, it generally must reach new:

hosts.

Transmission may occur through:

air → food/water → direct contact → body fluids → vectors

Different transmission strategies suit different pathogens.

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Reservoirs

A reservoir is a place or population in which a pathogen normally lives and can persist.

Reservoirs may include:

• humans
• other animals
• soil
• water

A reservoir can provide a continuing source of:

infection.

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Zoonotic Diseases

A zoonotic disease is an infectious disease that can be transmitted between animals and:

humans.

The pathogens involved may include:

• viruses
• bacteria
• parasites
• fungi

Animal hosts can sometimes act as reservoirs for pathogens that later infect humans.

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Preventing Pathogen Transmission

Disease prevention often focuses on interrupting:

transmission.

Methods can include:

• handwashing
• safe food handling
• clean drinking water
• sanitation
• appropriate ventilation
• vaccination
• vector control
• safe medical practices

The most appropriate method depends on how a particular pathogen spreads.

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Antibiotics

Antibiotics are medicines used to treat certain:

bacterial infections.

They do not work against:

viruses.

Different antibiotics target particular bacterial structures or processes.

Incorrect or unnecessary antibiotic use can contribute to:

antibiotic resistance.

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Vaccination

Vaccines prepare the immune system to recognize particular:

pathogens or pathogen components.

If the vaccinated person later encounters the pathogen, the immune system may respond more quickly and effectively.

Vaccination can reduce the likelihood or severity of certain infectious:

diseases.

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Worked Example 1

A bacterium lives harmlessly in a person's digestive system.

Is it necessarily a pathogen?

No.

Many bacteria are non-pathogenic or beneficial.

A microorganism is considered pathogenic when it has the ability to:

cause disease under relevant conditions.

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Worked Example 2

A virus enters a respiratory cell and uses the cell to produce new virus particles.

What is the host?

The:

infected organism, with its cells serving as the sites of viral replication.

What is the pathogen?

The:

virus.

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Worked Example 3

A mosquito carries Plasmodium from one person to another.

Identify the pathogen.

Plasmodium

Identify the vector.

Mosquito

Identify the host.

The infected human is one of the parasite's hosts; Plasmodium also develops in the mosquito during its life cycle.

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Worked Example 4

A bacterium produces a toxin that damages intestinal cells.

How does the pathogen cause disease?

It produces a harmful substance that interferes with normal:

cell and tissue function.

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Worked Example 5

A person encounters a pathogen but does not become infected.

Give a possible explanation.

The person's defenses may have removed or destroyed the pathogen before it could establish an:

infection.

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Worked Example 6

A person becomes infected but has no symptoms.

Does this mean the pathogen is absent?

No.

An infection can be:

asymptomatic.

The pathogen may be present even though the person does not feel ill.

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Common Mistake: All Bacteria Are Harmful

Most bacteria are not:

pathogens.

Many are harmless, and many perform beneficial ecological or biological functions.

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Common Mistake: All Microorganisms Cause Disease

A microorganism is not automatically a:

pathogen.

Many microorganisms do not cause disease.

Some are essential for ecosystems and normal biological processes.

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Common Mistake: Viruses Are Bacteria

Viruses and bacteria are fundamentally different.

Bacteria are:

cells.

Viruses are:

non-cellular infectious agents that depend on host cells for replication.

This is one reason antibiotics do not treat viral infections.

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Common Mistake: The Vector Is the Pathogen

A vector carries or transmits a:

pathogen.

For malaria:

Plasmodium = pathogen

mosquito = vector

They are not the same thing.

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Common Mistake: Infection Always Causes Symptoms

Some infections are:

asymptomatic.

A person may carry and sometimes transmit a pathogen without showing obvious symptoms.

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Common Mistake: Pathogens Always Kill Their Hosts

Most pathogens do not inevitably kill their:

hosts.

The outcome depends on the pathogen, the host, treatment, immune response, and many other factors.

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Check Your Understanding

1. Define a pathogen.

2. Define a host.

3. What is an infection?

4. Explain the difference between infection and disease.

5. Are all microorganisms pathogens? Explain.

6. Give two examples of useful roles performed by non-pathogenic microorganisms.

7. Name four major categories of pathogens.

8. Describe the basic structure of a bacterium.

9. How can pathogenic bacteria cause disease?

10. What is a toxin?

11. Describe the basic structure of a virus.

12. Why do viruses require host cells?

13. Why do antibiotics not treat viral infections?

14. Give two examples of viral diseases.

15. What is a fungal pathogen?

16. Give an example of a fungal disease.

17. What is a protozoan pathogen?

18. What type of pathogen causes malaria?

19. What is a vector?

20. Distinguish between a pathogen and a vector.

21. Give three possible routes by which pathogens can enter the body.

22. Explain respiratory transmission.

23. Explain how contaminated food or water can spread disease.

24. What is colonization?

25. Name three defenses the human body has against pathogens.

26. Describe three ways pathogens can cause disease.

27. Explain how the immune response can contribute to disease symptoms.

28. Why does exposure to a pathogen not always result in infection?

29. Why does infection not always result in symptoms?

30. What is an asymptomatic infection?

31. Define pathogenicity.

32. What is an opportunistic pathogen?

33. Give two factors that can affect host susceptibility.

34. What is meant by host range?

35. Explain the relationship between a pathogen and its host.

36. What is a reservoir?

37. What is a zoonotic disease?

38. Give four methods of reducing pathogen transmission.

39. Explain why "all microorganisms are harmful" is scientifically incorrect.

40. Explain how the pathogen, host, and environment can all influence whether disease develops.

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Key Terms

• Pathogen: Biological agent capable of causing disease in a host.
• Host: Organism in or on which a pathogen lives and may reproduce.
• Microorganism: Microscopic organism; viruses are commonly discussed alongside microorganisms but are not generally considered living organisms.
• Infection: Establishment and multiplication of a pathogen in a host.
• Disease: Disruption of normal biological function that may produce signs or symptoms.
• Pathogenic: Capable of causing disease.
• Non-pathogenic: Not normally capable of causing disease.
• Bacterium: Single-celled prokaryotic organism.
• Virus: Non-cellular infectious agent containing genetic material and requiring host cells for replication.
• Fungus: Eukaryotic organism; some species can act as pathogens.
• Protozoan: Usually single-celled eukaryotic organism; some species are parasitic.
• Parasite: Organism that obtains resources from a host, usually harming the host.
• Vector: Organism that transmits a pathogen between hosts.
• Toxin: Harmful substance produced by some organisms.
• Colonization: Establishment of a microorganism in or on a host.
• Transmission: Movement of a pathogen from one host or source to another.
• Reservoir: Population or environment in which a pathogen can persist.
• Zoonosis: Infectious disease transmitted between animals and humans.
• Pathogenicity: Ability to cause disease.
• Opportunistic pathogen: Microorganism that causes disease mainly under particular favorable conditions.
• Asymptomatic: Infected without noticeable symptoms.
• Immune response: Biological response used by the body to recognize and combat pathogens.

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Key Takeaways

• A pathogen is a biological agent capable of causing disease.
• Not all microorganisms are pathogens.
• Many microorganisms are harmless or beneficial.
• The four major pathogen groups commonly studied are bacteria, viruses, fungi, and protozoa.
• Bacteria are living prokaryotic cells.
• Some bacteria cause disease by damaging tissues or producing toxins.
• Viruses are non-cellular infectious agents.
• Viruses require host cells to reproduce.
• Antibiotics act against susceptible bacteria, not viruses.
• Some fungi can cause infections.
• Some protozoa are parasites that cause disease.
• Plasmodium is the pathogen responsible for malaria.
• A vector carries a pathogen between hosts.
• A host provides an environment in which a pathogen may live or reproduce.
• Pathogens can spread through air, food, water, contact, body fluids, or vectors.
• Pathogens can cause disease by damaging cells, producing toxins, taking resources, or triggering harmful host responses.
• Exposure does not always result in infection.
• Infection does not always result in noticeable disease.
• Some infections are asymptomatic.
• Host defenses influence whether infection becomes established.
• Pathogenicity depends on interactions between the pathogen and the host.
• Some microorganisms act as opportunistic pathogens only under certain conditions.
• The pathogen, host, and environment can all influence disease development.
• Understanding how pathogens spread helps us develop effective methods to prevent infectious disease.

What Is a Viral Disease?

A viral disease is an illness caused by infection with a:

virus.

Viruses are extremely small infectious agents. Unlike bacteria, viruses are not cells and cannot reproduce independently.

To make new viruses, they must infect a:

host cell.

Examples of viral diseases include:

• influenza
• COVID-19
• measles
• chickenpox
• dengue
• hepatitis B
• HIV infection and AIDS
• polio
• rabies

Different viruses infect different tissues and spread through different routes.

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What Is a Virus?

A virus consists of genetic material surrounded by a protective:

protein coat.

The genetic material may be:

DNA or RNA.

The protein coat is called a:

capsid.

Some viruses also possess an outer:

lipid envelope.

Viruses do not have the complete cellular machinery required to reproduce independently.

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Basic Virus Structure

A virus may contain:

Genetic material

DNA or RNA containing instructions for producing new virus particles.

Capsid

A protein coat surrounding and protecting the genetic material.

Envelope

Some viruses possess a lipid membrane surrounding the capsid.

Surface proteins

Proteins that can help a virus recognize and attach to particular host cells.

Different viruses have different shapes and structures.

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Are Viruses Living?

Viruses have some characteristics associated with living organisms, such as:

• genetic material
• ability to evolve
• ability to reproduce when inside suitable cells

However, they cannot independently:

• reproduce
• perform cellular metabolism
• maintain normal cellular functions

For this reason, viruses are generally described as:

non-cellular infectious agents,

rather than complete living cells.

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Viruses Need Host Cells

A virus cannot simply divide like a bacterium.

Instead, it must infect a:

host cell.

The virus uses the host cell's machinery and resources to produce:

viral components.

These components are assembled into new virus particles.

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Viral Replication

A simplified viral replication cycle is:

attachment → entry → replication and protein production → assembly → release

The exact process varies between different viruses.

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Step 1: Attachment

A virus first attaches to specific molecules on the surface of a:

host cell.

These molecules are often called:

receptors.

The interaction between viral surface proteins and cellular receptors helps determine which cells the virus can infect.

---

Step 2: Entry

The virus or its genetic material enters the:

host cell.

Different viruses use different mechanisms.

Some viruses enter the cell inside membrane-bound structures.

Others deliver their genetic material into the cell.

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Step 3: Viral Replication

Once inside the cell, viral genetic information directs the production of:

• new viral genetic material
• viral proteins

The virus depends heavily on the host cell's:

machinery.

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Step 4: Assembly

New viral components are assembled into:

virus particles.

Each new particle contains viral genetic material surrounded by the appropriate structural proteins.

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Step 5: Release

New viruses leave the infected cell.

Some viruses cause the cell to:

burst.

Others leave by budding through the cell membrane.

The released viruses can then infect:

other cells.

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How Viral Infections Cause Disease

Viral infections can cause disease in several ways.

Viruses may:

• damage infected cells
• destroy cells during replication
• interfere with normal cell functions
• trigger inflammation
• produce strong immune responses
• cause long-term changes in infected tissues

The exact mechanism depends on the:

virus.

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Viral Infections vs Bacterial Infections

Viruses and bacteria are fundamentally different.

Viruses

• are not cells
• contain DNA or RNA
• require host cells for replication
• are generally much smaller than bacteria
• are not treated with antibiotics

Bacteria

• are living prokaryotic cells
• contain DNA
• contain ribosomes
• can reproduce independently under suitable conditions
• reproduce mainly by binary fission
• some bacterial infections can be treated with antibiotics

This difference is extremely important when choosing:

treatment.

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Antibiotics and Viral Disease

Antibiotics do not treat viral infections.

Antibiotics target structures or processes found in bacteria, such as:

• bacterial cell walls
• bacterial ribosomes
• particular bacterial metabolic pathways

Viruses do not possess these structures in the same way.

Therefore, using antibiotics against a purely viral infection does not eliminate the virus.

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Can Antibiotics Ever Be Used During a Viral Illness?

Sometimes a viral infection is followed by a separate:

bacterial infection.

In that situation, antibiotics may be used to treat the bacterial infection.

The antibiotics are treating:

the bacteria, not the virus.

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Influenza

Influenza, commonly called the flu, is caused by:

influenza viruses.

It primarily infects the respiratory system.

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Influenza Symptoms

Common symptoms can include:

• fever
• cough
• sore throat
• muscle aches
• headache
• fatigue

Some people develop complications, particularly those at greater risk of severe illness.

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Influenza Transmission

Influenza spreads mainly through:

respiratory transmission.

Infected people release virus-containing respiratory particles when they:

• breathe
• talk
• cough
• sneeze

Transmission is more likely when susceptible people have sufficient exposure to infectious particles.

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Managing Influenza

Management may include:

• rest
• fluids
• symptom management
• antiviral medication in appropriate situations

Vaccination can reduce the risk of influenza and its complications.

Influenza vaccines are updated regularly because influenza viruses:

change over time.

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COVID-19

COVID-19 is caused by:

SARS-CoV-2.

SARS-CoV-2 is a:

coronavirus.

It primarily spreads through infectious respiratory particles.

---

COVID-19 Symptoms

Symptoms vary widely and may include:

• fever
• cough
• sore throat
• fatigue
• headache
• muscle aches
• changes in smell or taste

Some infections produce few or no noticeable symptoms.

Others can cause serious disease.

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Preventing Respiratory Viral Disease

Depending on the virus and situation, strategies can include:

• vaccination
• good ventilation
• staying away from others while infectious
• respiratory hygiene
• appropriate mask use in higher-risk settings
• hand hygiene

Effective prevention focuses on the virus's actual:

transmission route.

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Measles

Measles is caused by the measles virus.

It is a highly transmissible respiratory viral disease.

Symptoms commonly include:

• fever
• cough
• runny nose
• red eyes
• characteristic rash

Measles can sometimes cause serious complications.

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Measles Prevention

The most important method of preventing measles is:

vaccination.

The measles-containing vaccine produces immune memory that allows the immune system to respond much more effectively if the virus is encountered later.

High vaccination coverage also reduces opportunities for the virus to spread through:

populations.

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Chickenpox

Chickenpox is caused by:

varicella-zoster virus.

Common symptoms include:

• fever
• tiredness
• an itchy blister-like rash

After the initial infection, the virus can remain dormant in nerve tissue.

Years later it can sometimes reactivate and cause:

shingles.

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Dengue

Dengue is a viral disease caused by dengue viruses.

It is transmitted mainly by infected:

Aedes mosquitoes.

The mosquito is the:

vector.

The dengue virus is the:

pathogen.

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Dengue Transmission

The basic transmission cycle includes:

infected human → mosquito → another human

A mosquito can acquire dengue virus when feeding on an infected person.

After the virus develops within the mosquito, the mosquito can transmit it during a later:

bite.

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Dengue Prevention

Because mosquitoes transmit dengue, prevention focuses heavily on reducing contact with:

Aedes mosquitoes.

Strategies can include:

• removing standing water where mosquitoes breed
• using insect repellent
• using screens
• wearing protective clothing
• mosquito-control programs

This demonstrates why prevention strategies should match the pathogen's:

mode of transmission.

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HIV

HIV stands for:

Human Immunodeficiency Virus.

HIV infects important cells of the:

immune system.

Without effective treatment, the immune system can become progressively damaged.

---

HIV and AIDS

HIV and AIDS are not the same thing.

HIV is the virus.

AIDS is a late stage of HIV infection characterized by severe immune-system damage and particular clinical conditions.

Modern treatment can greatly reduce the likelihood that HIV infection progresses to:

AIDS.

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HIV Transmission

HIV can be transmitted through certain body fluids.

Important routes include:

• sexual transmission
• blood exposure
• sharing contaminated needles
• transmission during pregnancy, birth, or breastfeeding

HIV is not spread through ordinary casual contact such as:

• shaking hands
• sharing a room
• hugging
• ordinary social contact

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Managing HIV

HIV can be managed using:

antiretroviral therapy, or ART.

ART uses medicines that interfere with different stages of the HIV replication cycle.

Effective treatment can reduce the amount of virus in the body to very low levels and allows many people with HIV to live long, healthy lives.

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Hepatitis B

Hepatitis B is caused by the hepatitis B virus.

It primarily affects the:

liver.

Transmission can occur through:

• blood
• sexual contact
• transmission from parent to child during birth

Some infections become chronic.

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Preventing Hepatitis B

An effective:

vaccine

is available against hepatitis B.

Other prevention measures depend on reducing exposure to infected blood and body fluids.

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Polio

Polio, or poliomyelitis, is caused by:

poliovirus.

Most infections do not cause paralysis, but in a small proportion of cases the virus can affect the nervous system and cause:

paralysis.

Vaccination has dramatically reduced polio worldwide.

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Rabies

Rabies is a viral disease affecting the:

nervous system.

Humans usually become infected through saliva from an infected animal, often through a:

bite.

Once clinical symptoms appear, rabies is almost always fatal.

However, prompt post-exposure medical treatment can prevent disease after many exposures.

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Different Viruses Spread Differently

Viruses can spread through many routes.

These include:

• respiratory transmission
• direct contact
• fecal-oral transmission
• blood
• sexual contact
• vectors
• animal bites
• parent-to-child transmission

There is no single transmission route used by all:

viruses.

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Respiratory Transmission

Viruses such as influenza viruses and SARS-CoV-2 can spread through:

respiratory particles.

Transmission may be influenced by:

• proximity
• duration of exposure
• ventilation
• number of infectious people
• population immunity

Indoor environments with poor ventilation can increase opportunities for transmission.

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Fecal-Oral Transmission

Some viruses can spread through the:

fecal-oral route.

Examples include:

• norovirus
• poliovirus
• hepatitis A virus

Good sanitation, clean water, food hygiene, and handwashing can reduce this type of transmission.

---

Bloodborne Transmission

Some viruses can spread through:

blood.

Examples include:

• HIV
• hepatitis B virus
• hepatitis C virus

Prevention can include safe medical practices and avoiding shared contaminated needles.

---

Vector-Borne Transmission

Some viruses depend on:

vectors.

Examples include:

Dengue virus → Aedes mosquito

Yellow fever virus → mosquitoes

West Nile virus → mosquitoes

Controlling vectors can therefore reduce transmission.

---

How Viruses Spread Through Populations

For a virus to spread through a population, it must successfully move from infected hosts to:

susceptible hosts.

Population spread is affected by:

• transmission efficiency
• contact patterns
• infectious period
• population density
• immunity
• vaccination
• environmental conditions
• human behavior
• vector populations

These factors interact.

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Susceptible Hosts

A susceptible host is an individual who can become infected when exposed to a particular virus.

Susceptibility can be influenced by:

• previous infection
• vaccination
• immune function
• age
• characteristics of the virus

A virus spreads more easily when it frequently encounters susceptible:

hosts.

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Incubation Period

The incubation period is the time between:

infection and the appearance of symptoms.

Different viruses have different incubation periods.

For some viral diseases, a person may be infectious before noticeable symptoms appear.

---

Asymptomatic Infection

Some viral infections produce no noticeable:

symptoms.

This is called an:

asymptomatic infection.

Depending on the virus, an asymptomatic person may still be able to transmit the infection.

This can make disease control more difficult.

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The Infectious Period

The infectious period is the time during which an infected person can transmit a virus to:

others.

The infectious period may begin:

• before symptoms
• during symptoms
• sometimes after symptoms improve

The pattern depends on the particular virus.

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Outbreaks

An outbreak occurs when cases of a disease increase above what is normally expected in a particular place or group.

If a virus reaches many susceptible people and transmission continues, the number of cases can:

increase.

Public-health measures aim to interrupt transmission.

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Epidemics

An epidemic is an occurrence of disease cases above expected levels in a population or region.

The exact use of the term depends on:

context.

An epidemic may involve a particular city, region, country, or population.

---

Pandemics

A pandemic is an epidemic that spreads across multiple countries or continents and affects large numbers of people.

The word describes the:

geographic spread of disease.

It does not by itself describe how severe the disease is for each infected person.

---

Immunity and Viral Spread

The immune system can develop memory after:

• some natural infections
• vaccination

Immune memory may allow a faster response during later exposure.

Depending on the virus, immunity may reduce:

• infection
• disease severity
• infectiousness
• duration of infection

The strength and duration of protection vary between viruses.

---

Vaccines

A vaccine exposes the immune system to an antigen, or instructions for producing an antigen, in a controlled way.

This stimulates the development of:

immune memory.

If the actual virus is encountered later, the immune system can respond more rapidly.

---

Vaccines Do Not Directly Kill Viruses

Vaccines primarily work by preparing the:

immune system.

They are mainly:

preventive.

They help the body respond more effectively if exposure occurs later.

This is different from a medicine used to treat an infection that has already become established.

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Antiviral Medicines

Antiviral medicines interfere with stages of viral replication.

Depending on the virus, they may interfere with:

• viral entry
• genome replication
• viral enzymes
• assembly
• release

Antivirals are usually specific to particular viruses or groups of viruses.

---

Why Antivirals Are Challenging to Develop

Viruses reproduce inside:

host cells.

This makes treatment challenging because a medicine must interfere with the virus without causing unacceptable damage to the host's own cells.

Viruses can also:

mutate and evolve.

This may lead to antiviral resistance.

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Antiviral Treatment for Influenza

Certain antiviral medicines can be used against:

influenza.

They may reduce the duration or severity of illness when used appropriately, particularly when started early.

They do not replace:

vaccination.

Prevention and treatment serve different purposes.

---

Antiretroviral Therapy

HIV is treated using combinations of:

antiretroviral medicines.

Different drugs target different stages of HIV replication.

Using combinations makes it more difficult for the virus to develop resistance.

Treatment can suppress viral replication very effectively.

---

Supportive Care

Many viral illnesses are managed partly through:

supportive care.

Supportive care helps the body while the immune system responds.

Depending on the illness, this may include:

• rest
• fluids
• symptom management
• oxygen or other medical support in severe cases

Supportive care does not necessarily directly destroy the virus.

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The Immune System

For many viral infections, the body's:

immune system

is responsible for eventually controlling or eliminating the infection.

Important components include:

• innate immune defenses
• antibodies
• B cells
• T cells
• memory cells

The immune response can also contribute to some disease symptoms.

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Prevention vs Treatment

These two ideas should not be confused.

Prevention

Occurs mainly:

before infection or exposure leads to disease.

Examples:

• vaccination
• ventilation
• hand hygiene
• vector control
• safer sexual practices

Treatment

Occurs:

after infection has occurred.

Examples:

• antiviral medicines
• supportive care
• antiretroviral therapy

Some interventions can also be used shortly after exposure to prevent disease from developing.

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Prevention Must Match Transmission

The best prevention strategy depends on how the virus:

spreads.

For influenza:

vaccination + respiratory precautions

For dengue:

mosquito control + protection from mosquito bites

For hepatitis B:

vaccination + prevention of blood/body-fluid exposure

For HIV:

prevention of relevant blood and sexual exposure + effective medical prevention strategies

For rabies:

avoiding animal exposure + urgent post-exposure treatment when needed

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Comparing Influenza and Dengue

Influenza

Pathogen:

influenza virus

Transmission:

respiratory

Major prevention:

vaccination and reducing respiratory exposure

Dengue

Pathogen:

dengue virus

Transmission:

mosquito vector

Major prevention:

reducing mosquito exposure and controlling mosquito populations

The diseases are both viral, but their prevention strategies differ because their:

transmission routes differ.

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Comparing Measles and HIV

Measles

Main transmission:

respiratory

Important prevention:

vaccination

HIV

Main transmission:

specific body fluids

Important prevention:

reducing relevant exposure and using effective biomedical prevention strategies

Again, understanding transmission determines appropriate prevention.

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Viral Mutation

Viruses can acquire changes in their genetic material called:

mutations.

Most mutations do not automatically make a virus more dangerous.

However, some mutations can alter characteristics such as:

• transmissibility
• immune recognition
• drug susceptibility
• interaction with host cells

Evolution can therefore influence viral populations over time.

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Natural Selection in Viruses

Suppose a viral population contains genetic variation.

If environmental conditions favor one variant, that variant may reproduce more successfully.

Over generations:

advantageous inherited variants can become more common.

This is:

natural selection.

Viruses therefore evolve, just as other biological populations do.

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Antiviral Resistance

If a mutation allows a virus to reproduce despite an antiviral drug, the drug creates:

selection pressure.

Susceptible viruses are inhibited more strongly.

Resistant variants may continue reproducing.

Over time, resistance can become more common.

This is one reason antiviral medicines must be used:

appropriately.

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Why Prevention Matters

Preventing viral disease can:

• protect individuals
• reduce severe illness
• reduce transmission
• protect vulnerable populations
• reduce pressure on healthcare systems
• reduce opportunities for some viruses to spread

Preventing infection can be especially important when effective treatments are limited.

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Layered Prevention

Sometimes several prevention strategies are combined.

For respiratory viral disease, this might include:

vaccination + ventilation + staying away from others while infectious + appropriate respiratory precautions

Each layer may reduce some risk.

Together they can provide greater protection than relying on only one strategy.

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Worked Example 1

A pathogen contains RNA inside a protein coat and can reproduce only inside host cells.

What type of pathogen is it?

A:

virus.

The dependence on host cells is a major characteristic of viruses.

---

Worked Example 2

A patient has influenza.

Would an antibiotic kill the influenza virus?

No.

Influenza is caused by a virus.

Antibiotics target:

bacteria.

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Worked Example 3

A virus spreads when infected mosquitoes bite people.

What type of transmission is occurring?

Vector-borne transmission.

A prevention strategy should therefore include:

reducing contact with the vector.

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Worked Example 4

A vaccine causes the immune system to produce memory cells.

Is this prevention or treatment?

Primarily:

prevention.

The immune system is being prepared before future exposure.

---

Worked Example 5

A medicine blocks an enzyme needed for a virus to reproduce.

What type of medicine is this?

An:

antiviral.

It interferes with viral replication.

---

Worked Example 6

Two viral diseases are prevented using completely different methods.

Why?

Different viruses can have different:

transmission routes.

Prevention should target the way each virus spreads.

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Worked Example 7

A person is infected but has no symptoms.

Can the person necessarily be considered non-infectious?

No.

For some viral infections, asymptomatic people can still transmit the virus.

Infectiousness depends on the particular virus and stage of infection.

---

Worked Example 8

A viral disease is spreading rapidly in a poorly ventilated indoor environment.

What factor could be contributing to transmission?

If the virus spreads through respiratory particles:

poor ventilation can allow infectious particles to accumulate.

Improving ventilation may reduce exposure.

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Comparing Viral and Bacterial Disease

Structure

Virus:

non-cellular

Bacterium:

prokaryotic cell

Reproduction

Virus:

requires a host cell

Bacterium:

can reproduce independently under suitable conditions

Genetic material

Virus:

DNA or RNA

Bacterium:

DNA

Ribosomes

Virus:

absent

Bacterium:

present

Antibiotics

Virus:

not effective

Bacterium:

may be effective depending on susceptibility

Specialized medicines

Virus:

antivirals may be available

Bacterium:

antibiotics may be available

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Common Mistake: Viruses Are Tiny Bacteria

Viruses are not bacteria.

They have fundamentally different:

• structures
• replication mechanisms
• biological properties

A virus is:

not a cell.

A bacterium is:

a living prokaryotic cell.

---

Common Mistake: Antibiotics Treat Viral Disease

Antibiotics do not treat viral infections.

Unnecessary antibiotic use can contribute to:

antibiotic resistance in bacteria.

---

Common Mistake: Every Virus Has an Antiviral Treatment

Many viral infections do not have a specific antiviral medicine.

Management may rely on:

• immune response
• supportive care
• prevention

The available treatment depends on the particular virus.

---

Common Mistake: Vaccines Cure Existing Viral Infections

Vaccines are primarily designed to:

prevent disease or reduce its impact by preparing the immune system.

They are not generally treatments that eliminate an already established infection.

---

Common Mistake: All Viruses Spread Through the Air

Viruses use many different transmission routes.

Examples include:

Influenza → respiratory

Dengue → vector-borne

HIV → particular body fluids

Poliovirus → mainly fecal-oral

Rabies → infected animal saliva, usually through bites

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Common Mistake: Infection Always Means Symptoms

Viral infections can be:

asymptomatic.

A person can be infected without feeling ill.

Depending on the virus, that person may still be capable of transmission.

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Common Mistake: Vaccination Guarantees No Infection

Vaccines differ in how effectively they prevent:

• infection
• symptoms
• severe disease
• transmission

Protection is not necessarily absolute.

However, vaccines can substantially reduce disease risk and complications for many viral infections.

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Check Your Understanding

1. Define a viral disease.

2. What is a virus?

3. Name the two main components found in all viruses.

4. What additional structure surrounds some viruses?

5. Why can't viruses reproduce independently?

6. Describe the main stages of viral replication.

7. What happens during viral attachment?

8. How can viruses damage host cells?

9. Give five examples of viral diseases.

10. Explain one major difference between viruses and bacteria.

11. How do bacteria reproduce?

12. How do viruses reproduce?

13. Why do antibiotics not treat viral infections?

14. What virus causes influenza?

15. How does influenza mainly spread?

16. Give three common influenza symptoms.

17. What virus causes COVID-19?

18. What is the main method of preventing measles?

19. What virus causes chickenpox?

20. What can varicella-zoster virus cause later in life?

21. What type of virus causes dengue?

22. What is the vector responsible for most dengue transmission?

23. Distinguish between the dengue pathogen and its vector.

24. What does HIV attack?

25. Explain the difference between HIV and AIDS.

26. Give three routes through which HIV can be transmitted.

27. What is antiretroviral therapy?

28. What organ is primarily affected by hepatitis B?

29. How can rabies commonly be transmitted?

30. What is an incubation period?

31. What is an asymptomatic infection?

32. What is an infectious period?

33. Give four factors that can influence viral spread through a population.

34. How does vaccination help prevent viral disease?

35. What is an antiviral medicine?

36. Why can developing antiviral medicines be difficult?

37. Compare prevention and treatment of viral disease.

38. Explain why different viral diseases require different prevention strategies.

39. Explain how mutation and natural selection can change viral populations.

40. Compare bacterial and viral infections in terms of structure, reproduction, transmission, prevention, and treatment.

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Key Terms

• Virus: Non-cellular infectious agent that requires a host cell to reproduce.
• Viral disease: Disease caused by infection with a virus.
• Host cell: Cell used by a virus for replication.
• Capsid: Protein coat surrounding viral genetic material.
• Viral genome: Viral genetic material consisting of DNA or RNA.
• Envelope: Lipid membrane surrounding some viruses.
• Receptor: Molecule that can allow a virus to recognize and attach to a host cell.
• Viral replication: Process by which new virus particles are produced inside host cells.
• Antiviral: Medicine that interferes with viral replication.
• Antiretroviral therapy: Combination of medicines used to suppress HIV replication.
• Supportive care: Treatment that supports body functions without necessarily directly eliminating the virus.
• Vaccination: Method of preparing the immune system to respond to a pathogen or antigen.
• Immune memory: Ability of the immune system to respond more rapidly after previous exposure to an antigen.
• Vector: Organism that transmits a pathogen between hosts.
• Respiratory transmission: Spread through infectious respiratory particles.
• Incubation period: Time between infection and development of symptoms.
• Infectious period: Period during which an infected host can transmit the virus.
• Asymptomatic infection: Infection without noticeable symptoms.
• Outbreak: Increase in disease cases above expected levels in a particular setting.
• Epidemic: Disease occurrence above expected levels within a population or region.
• Pandemic: Epidemic extending across multiple countries or continents.
• Mutation: Change in genetic material.
• Antiviral resistance: Ability of a virus to reproduce despite an antiviral medicine that would normally inhibit it.

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Key Takeaways

• Viral diseases are caused by viruses.
• Viruses are fundamentally different from bacteria.
• Viruses are not cells and must use host cells to reproduce.
• Viral genetic material can be DNA or RNA.
• A protein capsid surrounds the viral genome.
• Some viruses also have an envelope.
• Viral replication involves attachment, entry, production of viral components, assembly, and release.
• Viruses can damage cells and trigger immune responses that contribute to symptoms.
• Examples of viral diseases include influenza, COVID-19, measles, chickenpox, dengue, HIV infection, hepatitis B, polio, and rabies.
• Different viruses use different transmission routes.
• Viral diseases may spread through respiratory particles, body fluids, contaminated food or water, vectors, or animal bites.
• Some viral infections can be asymptomatic.
• Viruses can sometimes spread before symptoms appear.
• Population spread depends on contact patterns, immunity, vaccination, environment, behavior, and characteristics of the virus.
• Antibiotics do not treat viral infections.
• Some viral diseases can be treated with specific antiviral medicines.
• Many viral illnesses also require supportive care.
• Vaccines prepare the immune system before future exposure.
• Prevention and treatment are different strategies.
• Prevention must match the virus's transmission route.
• Respiratory viruses may be controlled partly through vaccination and reducing respiratory exposure.
• Vector-borne viruses may require vector control.
• Bloodborne and sexually transmitted viruses require strategies that reduce relevant body-fluid exposure.
• Viruses can mutate and evolve through natural selection.
• Antiviral resistance can develop when resistant viral variants are selected.
• Combining appropriate prevention strategies can reduce transmission.
• Understanding the differences between viral and bacterial infections is essential for choosing appropriate prevention and treatment strategies.

What Are Emerging Diseases?

An emerging infectious disease is a disease that:

• appears in a population for the first time, or
• previously existed but is rapidly increasing in the number of cases, or
• is spreading into new geographic areas.

Emerging diseases are important because populations may have little immunity, treatments or vaccines may be limited, and health authorities may initially know relatively little about the pathogen.

A simple way to think about emergence is:

new pathogen OR new population OR increasing cases OR expanding geographic range

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What Are Re-emerging Diseases?

A re-emerging disease is a known disease that had previously declined or been controlled but begins to:

increase again.

Re-emergence may occur because of:

• falling vaccination rates
• antimicrobial resistance
• changes in pathogen populations
• breakdowns in public-health systems
• war or displacement
• environmental change
• changing human behaviour
• movement of infected people
• changes in vector populations

A disease does not have to be completely new to become a major public-health threat.

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Emerging vs Re-emerging

Emerging disease

A disease that is newly recognized, newly introduced into a population, rapidly increasing, or expanding geographically.

Re-emerging disease

A known disease that becomes important again after previously declining or being controlled.

The key difference is:

Emerging = new or expanding threat

Re-emerging = known threat returning or increasing

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Examples

Diseases that have been important examples of emergence or changing geographic spread include:

• COVID-19
• mpox
• Ebola virus disease
• Nipah virus infection
• avian influenza infections in humans
• Zika virus disease

Diseases that may re-emerge in particular populations include:

• measles
• tuberculosis
• cholera
• polio
• dengue

Whether a disease is described as emerging or re-emerging can depend on the:

time, place, and population being considered.

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Where Do New Infectious Diseases Come From?

Many emerging infectious diseases are:

zoonotic.

A zoonotic disease is an infectious disease that can pass between animals and humans.

Human activities that increase contact among wildlife, livestock, and people can increase opportunities for pathogens to cross species barriers. WHO identifies land-use change, habitat disruption, climate change, migration, trade, and other ecological pressures as factors affecting infectious-disease patterns.

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Spillover

Spillover occurs when a pathogen normally associated with one host species successfully infects another species.

For example:

animal reservoir → pathogen exposure → human infection

A single spillover event does not necessarily cause an epidemic.

For widespread human transmission to occur, the pathogen usually must be capable of continuing to spread effectively among susceptible hosts.

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Animal Reservoirs

A reservoir is a population or environment in which a pathogen can persist.

Possible reservoirs include:

• wildlife
• livestock
• humans
• water
• soil

Some emerging pathogens circulate naturally in animal populations before occasionally infecting:

humans.

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Why Zoonotic Diseases Matter

Humans and animals interact in many ways:

• agriculture
• hunting
• food production
• wildlife trade
• habitat expansion
• animal markets
• pet ownership
• livestock farming

These interactions can create opportunities for pathogens to cross between:

species.

This does not mean that contact with animals automatically causes disease emergence.

Emergence depends on many interacting biological, environmental, and social factors.

---

Factors That Contribute to Disease Emergence

Emerging diseases rarely have a single cause.

Important factors include:

1. Pathogen evolution

2. Zoonotic spillover

3. Land-use change

4. Climate change

5. Urbanization

6. Global travel

7. International trade

8. Population growth

9. Changes in agriculture

10. Antimicrobial resistance

11. Breakdown of public-health systems

WHO identifies environmental change, human and animal population changes, pathogen changes, and agricultural practices among the factors contributing to emerging diseases.

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Pathogen Evolution

Pathogen populations can change genetically through:

mutation and other genetic processes.

Natural selection can then alter the frequency of different variants.

Changes may affect characteristics such as:

• transmission
• host range
• immune recognition
• drug resistance

Evolution does not have a goal.

Mutations occur, and environmental conditions determine which variants reproduce most successfully.

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Mutation

A mutation is a change in genetic material.

Mutations occur naturally.

Most mutations do not automatically make a pathogen:

more dangerous.

Some may have little effect.

Some may be harmful to the pathogen.

Occasionally, a mutation may give a pathogen an advantage under particular environmental conditions.

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Recombination

Some viruses can exchange or combine genetic material through processes such as:

recombination.

For example, WHO reported recombinant mpox viruses containing genomic elements from clades Ib and IIb in 2026. Recombination can occur when related viruses infect the same individual and exchange genetic material.

This illustrates how pathogen populations can continue changing after a disease has already emerged.

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Land-Use Change

Humans continually modify natural environments through:

• deforestation
• farming
• road construction
• mining
• urban expansion
• settlement

These activities can change interactions among:

humans + livestock + wildlife.

Increased contact may create new opportunities for pathogen transmission.

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Deforestation

Deforestation can alter:

• animal habitats
• animal movement
• biodiversity
• human-wildlife contact
• vector populations

For example, when humans expand into previously forested environments, people and livestock may encounter wildlife species more frequently.

This can change opportunities for:

zoonotic transmission.

---

Agriculture

Modern agriculture can involve large populations of:

animals living close together.

This may create opportunities for some pathogens to:

• spread between animals
• evolve
• encounter humans

Biosecurity, animal surveillance, vaccination where appropriate, and good farming practices can reduce these risks.

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Climate Change

Climate can influence infectious disease by affecting:

• vector distribution
• vector breeding
• pathogen survival
• rainfall
• temperature
• human migration
• ecosystems

WHO notes that climate can strongly influence the distribution and population sizes of disease vectors.

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Vector-Borne Disease and Climate

Vectors such as:

• mosquitoes
• ticks
• fleas

depend on environmental conditions.

Changes in temperature and rainfall can sometimes alter where these vectors can:

survive and reproduce.

This may change the geographic distribution or seasonal transmission of diseases such as:

• dengue
• malaria
• West Nile virus disease

Climate is one factor among several; human behaviour, land use, public-health measures, and vector control also matter.

---

Urbanization

Rapid urbanization can create conditions that affect disease transmission.

These may include:

• high population density
• crowded housing
• inadequate sanitation
• standing water
• increased transportation
• close contact among many people

Cities can therefore create efficient transmission networks for some pathogens.

---

Population Growth

As populations grow, humans may increasingly:

• enter wildlife habitats
• expand agricultural land
• increase food production
• travel
• live in densely populated areas

These changes can create more opportunities for pathogen transmission.

---

Antimicrobial Resistance

Antimicrobial resistance, or AMR, occurs when microorganisms evolve so that medicines that previously controlled them become less effective.

This can involve resistance to:

• antibiotics
• antivirals
• antifungals
• antiparasitic medicines

AMR is a major global health challenge affecting humans, animals, food systems, and the environment.

---

How Resistance Develops

Suppose a bacterial population contains genetic variation.

An antibiotic is used.

Susceptible bacteria are killed or inhibited.

Resistant bacteria survive.

They reproduce.

Therefore:

variation → selection pressure → survival → reproduction → increasing resistance

This is an example of:

natural selection.

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Global Travel

Modern transportation allows people to travel between continents in:

hours.

An infected person may travel before realizing that they are ill.

WHO notes that billions of airline passenger journeys occur each year, increasing opportunities for infectious agents to spread internationally.

---

Incubation Period and Travel

The incubation period is the time between infection and the development of:

symptoms.

Imagine that a person becomes infected on Monday.

The person flies internationally on Tuesday.

Symptoms begin on Thursday.

During the journey, the person may not have known that they were:

infected.

For diseases that can be transmitted before symptoms appear, this creates an additional challenge.

---

Global Connectivity

A pathogen no longer needs months or years to move between distant populations.

International networks connect:

cities → countries → continents.

Therefore, an outbreak that begins locally can potentially become an international concern if transmission continues.

Global travel does not create the pathogen, but it can greatly increase the speed at which an established infection reaches:

new locations.

---

International Trade

Goods, food, plants, animals, and biological materials also move internationally.

Trade can sometimes transport:

• pathogens
• infected animals
• contaminated foods
• disease vectors

Global food production and distribution can therefore contribute to international outbreaks when contamination occurs.

---

Recent Outbreak Example: Mpox

Mpox is caused by monkeypox virus.

A multinational outbreak began in 2022, with cases occurring in countries that had not previously documented sustained mpox transmission. More recently, clade Ib caused outbreaks in previously unaffected areas and spread internationally. WHO continued publishing global mpox situation reports through 2026.

---

How Mpox Spreads

Mpox spreads mainly through:

close contact.

Transmission can involve contact with:

• skin lesions
• body fluids
• mucosal surfaces
• respiratory particles
• contaminated materials

In areas where animals carry the virus, animal-to-human transmission can also occur.

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Why Mpox Is an Important Example

Mpox demonstrates several features relevant to emerging disease:

• changes in geographic distribution
• international transmission
• multiple viral clades
• continued pathogen evolution
• importance of surveillance
• need for rapid diagnosis
• importance of contact tracing
• need for international cooperation

It demonstrates that an existing disease can become an:

emerging international threat.

---

Recent Outbreak Example: Marburg Virus Disease

Marburg virus disease is a severe viral disease.

It was first identified in 1967, so it is not a newly discovered disease.

However, outbreaks continue to occur in new places and populations.

Recent outbreaks included Equatorial Guinea and Tanzania in 2023, Rwanda in 2024, and Tanzania again in 2025.

---

Why Marburg Is Difficult to Control

Challenges include:

• severe disease
• need for rapid diagnosis
• potential healthcare-associated transmission
• need for contact tracing
• fear and misinformation
• limited specific treatment options

As of WHO's January 2025 overview, no approved specific antiviral treatment or vaccine was available, although candidates were under development.

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Other Recent Disease Events

WHO's 2025–2026 emergency-event reporting has included events involving:

• Ebola disease
• Nipah virus infection
• avian influenza
• mpox
• measles
• cholera
• vaccine-derived poliovirus
• anthrax

This shows that disease emergence and re-emergence are continuing global processes rather than rare events belonging only to the past.

---

Avian Influenza

Avian influenza refers to influenza viruses primarily adapted to birds.

Occasionally, some avian influenza viruses infect:

humans or other mammals.

These infections are closely monitored because influenza viruses can change genetically and may cross species barriers.

This is an example of why surveillance of animal disease can help protect:

human health.

---

Nipah Virus

Nipah virus is a zoonotic virus.

Fruit bats are its natural reservoir.

Human infections can occur through routes that have included:

• animal-to-human transmission
• contaminated food
• person-to-person transmission

Nipah is closely monitored because outbreaks can cause severe disease.

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Measles as a Re-emerging Disease

Measles is not a new disease.

It can nevertheless:

re-emerge.

If vaccination coverage decreases, susceptible people accumulate in a population.

An infected person can then introduce the virus.

Because measles spreads very efficiently, outbreaks can occur.

The sequence may look like:

declining vaccination → more susceptible people → virus introduced → rapid transmission → outbreak

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Cholera as a Re-emerging Disease

Cholera can return or increase where conditions allow transmission.

Important factors include:

• unsafe drinking water
• poor sanitation
• displacement
• damaged infrastructure
• overcrowding
• humanitarian emergencies

Controlling cholera therefore requires more than simply treating infected individuals.

The environmental conditions that permit transmission must also be addressed.

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Disease X

Public-health planning sometimes uses the term:

Disease X.

Disease X does not refer to one known pathogen.

It represents the possibility that a currently unknown pathogen could cause a serious future epidemic.

The concept emphasizes the need to prepare systems before the next major pathogen is:

identified.

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Detecting an Emerging Disease

Early detection is essential.

Health authorities may use:

• disease surveillance
• hospital reports
• laboratory testing
• genomic sequencing
• wastewater surveillance
• animal surveillance
• contact tracing
• international reporting systems

The goal is to identify unusual patterns before transmission becomes much larger.

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Disease Surveillance

Surveillance is the systematic collection and analysis of health information.

Scientists may look for:

• unusual clusters of illness
• unexpected deaths
• new symptoms
• changes in disease frequency
• unusual laboratory results
• infections appearing in new locations

Surveillance provides an:

early-warning system.

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Genomic Surveillance

Scientists can sequence the genetic material of pathogens.

This allows researchers to:

• identify pathogens
• compare samples
• track variants
• investigate transmission
• detect mutations
• study pathogen evolution

Genomic surveillance has become an important tool in modern outbreak investigation.

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Contact Tracing

Contact tracing attempts to identify people who may have been exposed to an infected individual.

A simplified process is:

case identified → contacts identified → contacts informed and assessed → appropriate monitoring/testing/actions

Contact tracing can interrupt transmission when cases and contacts can be identified sufficiently quickly.

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Challenges of Controlling Emerging Diseases

Emerging diseases are particularly difficult because scientists may initially not know:

• what the pathogen is
• where it came from
• how it spreads
• how long incubation lasts
• when infected people are infectious
• which populations are most vulnerable
• which treatments work
• whether previous immunity exists

Scientists must make decisions while evidence is still:

developing.

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Challenge 1: Delayed Detection

An outbreak may begin before health authorities recognize that something unusual is:

happening.

Early cases may resemble common illnesses.

By the time a new pathogen is identified, transmission may already have occurred.

---

Challenge 2: Asymptomatic Transmission

Some infections produce few or no symptoms.

If infected people can still transmit the pathogen, identifying cases becomes:

more difficult.

Symptom screening alone may miss some infectious individuals.

---

Challenge 3: Limited Testing

When a pathogen first emerges, there may initially be no widely available:

diagnostic test.

Scientists must first identify the pathogen and develop reliable testing methods.

Testing capacity must then be expanded.

---

Challenge 4: Limited Treatments

A newly emerging pathogen may have no specific:

medicine.

Treatment may initially depend largely on supportive care.

Developing and testing new treatments takes time.

---

Challenge 5: Limited Vaccines

A newly identified pathogen may have no available:

vaccine.

Scientists must:

identify targets → design candidates → test safety → test effectiveness → manufacture → distribute

Each stage requires scientific evidence and resources.

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Challenge 6: Pathogen Evolution

Pathogens continue to:

evolve.

New variants may differ in:

• transmissibility
• immune recognition
• drug susceptibility
• host range

Surveillance therefore needs to continue even after an outbreak is initially understood.

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Challenge 7: Misinformation

During outbreaks, inaccurate information can spread rapidly.

Misinformation can affect:

• health behaviour
• treatment decisions
• vaccination decisions
• trust
• cooperation with health authorities

Clear communication is therefore an important part of disease control.

---

Challenge 8: Limited Healthcare Resources

Large outbreaks can increase demand for:

• hospital beds
• healthcare workers
• diagnostic tests
• medicines
• protective equipment

If demand exceeds available resources, healthcare systems can become:

overwhelmed.

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Challenge 9: Inequality

Countries and communities do not have equal access to:

• laboratories
• vaccines
• medicines
• healthcare workers
• surveillance systems
• sanitation
• clean water

An outbreak may therefore be much harder to control in some populations than in others.

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Challenge 10: International Coordination

Pathogens do not respect:

national borders.

Effective control may require countries to share:

• outbreak information
• laboratory findings
• genomic data
• research
• medical resources
• surveillance information

WHO emphasizes coordinated international surveillance and response as important for emerging diseases.

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One Health

An important approach to emerging disease is called:

One Health.

One Health recognizes that the health of:

humans + animals + ecosystems

is interconnected.

WHO and its international partners emphasize this approach for emerging infectious diseases, antimicrobial resistance, environmental degradation, biodiversity loss, and climate-related health threats.

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Why One Health Matters

Imagine that scientists monitor only:

human disease.

They might miss changes occurring in:

• wildlife pathogens
• livestock infections
• mosquito populations
• antimicrobial resistance
• ecosystems

Monitoring human, animal, and environmental health together can provide earlier warning of emerging threats.

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Preventing Emerging Diseases

It is impossible to guarantee that no new infectious disease will ever emerge.

However, risk can be reduced through:

• disease surveillance
• animal-health monitoring
• vaccination
• sanitation
• clean water
• infection prevention
• responsible antimicrobial use
• vector control
• laboratory capacity
• genomic surveillance
• international cooperation
• rapid outbreak investigation

Preparedness is important because the next emerging pathogen cannot always be predicted.

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Worked Example 1

A disease has never previously been detected in humans.

Several human cases suddenly appear.

This could be described as:

an emerging disease.

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Worked Example 2

A disease was previously well controlled in a region.

Vaccination coverage decreases.

Several years later, large outbreaks occur.

This is an example of:

re-emergence.

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Worked Example 3

A virus normally circulates in wildlife.

Habitat change increases contact between wildlife and humans.

The virus infects a human.

This is an example of:

zoonotic spillover.

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Worked Example 4

A person becomes infected in one country.

They fly internationally during the incubation period.

Symptoms appear after arrival.

This demonstrates how:

global travel can move pathogens between distant populations before infection is recognized.

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Worked Example 5

A mosquito species expands into a new geographic region.

A virus transmitted by that mosquito begins appearing in the same region.

What factor may have contributed?

A change in:

vector distribution.

Environmental conditions, travel, trade, and other factors would also need investigation.

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Worked Example 6

Doctors discover that a bacterial infection is no longer responding to an antibiotic that previously worked.

One possible explanation is:

antimicrobial resistance.

Natural selection may have increased the proportion of resistant bacteria.

---

Worked Example 7

An outbreak begins, but scientists do not yet know how the disease spreads.

Why is control difficult?

Without knowing the:

mode of transmission,

it is difficult to know which interventions will most effectively interrupt spread.

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Worked Example 8

Scientists monitor infections in humans, livestock, wildlife, and vectors.

What approach does this represent?

One Health.

It recognizes connections among human, animal, and environmental health.

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Emerging Disease Investigation

When scientists investigate an unusual outbreak, they may ask:

Who is becoming ill?

Where are cases occurring?

When did cases begin?

What symptoms occur?

What pathogen is responsible?

How is it transmitted?

Is there an animal reservoir?

What are the risk factors?

How quickly is it spreading?

What interventions reduce transmission?

These questions help scientists move from:

observation → evidence → understanding → control.

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Common Mistake: Emerging Means Newly Evolved

An emerging disease does not necessarily involve a completely new:

pathogen.

A known pathogen can become emerging if it rapidly expands into new populations or geographic regions.

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Common Mistake: Re-emerging Means the Disease Disappeared Completely

A disease may have remained present at low levels.

If cases begin increasing substantially again, it can still be described as:

re-emerging.

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Common Mistake: All Emerging Diseases Come from Animals

Many important emerging infectious diseases are zoonotic, but disease emergence can also result from:

• pathogen evolution
• antimicrobial resistance
• changing transmission patterns
• environmental changes
• breakdown of disease-control programs

Zoonotic spillover is only one mechanism.

---

Common Mistake: Global Travel Creates New Diseases

Travel does not necessarily create a new:

pathogen.

Instead, travel can allow an existing pathogen to reach new populations very quickly.

---

Common Mistake: Every Mutation Makes a Pathogen More Dangerous

Most mutations do not automatically increase:

danger.

Mutations can be:

• neutral
• harmful to the pathogen
• occasionally advantageous

Evolution depends on selection acting on genetic variation.

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Common Mistake: Outbreak and Pandemic Mean the Same Thing

They describe different scales.

Outbreak: unusual increase in cases in a particular setting.

Epidemic: disease occurrence above expected levels in a population or region.

Pandemic: epidemic spreading across multiple countries or continents.

These terms mainly describe patterns of:

disease occurrence and spread.

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Check Your Understanding

1. Define an emerging infectious disease.

2. Define a re-emerging disease.

3. Explain the difference between emerging and re-emerging diseases.

4. Give three examples of diseases that have shown emergence or geographic expansion.

5. Give three examples of diseases that can re-emerge.

6. What is a zoonotic disease?

7. What is spillover?

8. What is an animal reservoir?

9. Explain how land-use change can contribute to disease emergence.

10. How can deforestation change disease risk?

11. Explain how agriculture can create opportunities for pathogen transmission.

12. How can climate influence vector-borne disease?

13. Explain how urbanization may influence disease spread.

14. What is antimicrobial resistance?

15. Explain how natural selection can produce increasing antimicrobial resistance.

16. How can global air travel influence infectious-disease spread?

17. Why is the incubation period important when considering international travel?

18. How can international trade contribute to disease spread?

19. Why is mpox an important example when studying emerging disease?

20. What is a disease vector?

21. Why are avian influenza viruses closely monitored?

22. Explain how measles can re-emerge.

23. How can breakdowns in sanitation contribute to re-emerging disease?

24. What is Disease X?

25. What is disease surveillance?

26. What is genomic surveillance?

27. How can genomic sequencing help investigate outbreaks?

28. What is contact tracing?

29. Why can delayed detection make an outbreak difficult to control?

30. Why can asymptomatic transmission create challenges?

31. Why might diagnostic testing initially be limited during a new outbreak?

32. Why can developing a vaccine take time?

33. How can pathogen evolution complicate disease control?

34. How can misinformation affect outbreak control?

35. Why can emerging diseases place pressure on healthcare systems?

36. Why is international cooperation important?

37. What is the One Health approach?

38. Explain why animal-health surveillance can protect human health.

39. Suggest five strategies that could improve preparedness for emerging diseases.

40. An unknown infectious disease begins appearing in several countries. Describe how scientists could investigate the disease and develop strategies to reduce its spread.

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Key Terms

• Emerging disease: Disease that appears for the first time or rapidly increases in incidence or geographic range.
• Re-emerging disease: Known disease that increases again after previously declining or being controlled.
• Zoonosis: Infectious disease transmitted between animals and humans.
• Spillover: Transmission of a pathogen from its usual host species into a new species.
• Reservoir: Population or environment in which a pathogen normally persists.
• Vector: Organism that transmits a pathogen between hosts.
• Mutation: Change in genetic material.
• Recombination: Process in which genetic material is exchanged or combined.
• Antimicrobial resistance: Ability of microorganisms to survive medicines that previously controlled them.
• Incubation period: Time between infection and development of symptoms.
• Surveillance: Systematic collection and analysis of disease information.
• Genomic surveillance: Monitoring pathogens using genetic sequence information.
• Contact tracing: Identification and follow-up of people who may have been exposed to an infected individual.
• Outbreak: Increase in disease cases above expected levels in a particular setting.
• Epidemic: Disease occurrence above expected levels within a population or region.
• Pandemic: Epidemic spreading across multiple countries or continents.
• One Health: Approach recognizing the connections among human, animal, and environmental health.
• Disease X: Concept representing a serious epidemic that could be caused by a currently unknown pathogen.

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Key Takeaways

• Emerging diseases are new, rapidly increasing, or geographically expanding infectious diseases.
• Re-emerging diseases are known diseases that begin increasing again after previously declining.
• Many emerging infectious diseases involve zoonotic transmission.
• Spillover occurs when a pathogen crosses from its usual host species into another species.
• Pathogen evolution can contribute to disease emergence.
• Land-use change and habitat disruption can alter interactions among humans, livestock, wildlife, and pathogens.
• Climate can affect vectors and patterns of infectious-disease transmission.
• Urbanization and population growth can create new opportunities for transmission.
• Antimicrobial resistance can cause previously treatable infections to become harder to control.
• Modern international travel can transport infections between continents within hours.
• International trade can also contribute to the movement of pathogens and vectors.
• Recent and continuing outbreak concerns include mpox, avian influenza, Nipah virus infection, Ebola and Marburg virus disease, while diseases such as measles, cholera, and polio can re-emerge in particular populations.
• Disease surveillance provides early warning of unusual outbreaks.
• Genomic sequencing can identify pathogens and track their evolution.
• Contact tracing can help interrupt transmission.
• Emerging diseases are difficult to control because information is often incomplete during the early stages.
• Vaccines, treatments, and diagnostic tests may initially be unavailable.
• Asymptomatic or presymptomatic transmission can make detection more difficult.
• Misinformation and unequal healthcare resources can complicate outbreak responses.
• International cooperation is important because infectious diseases can cross national borders.
• One Health connects human, animal, and environmental health.
• Preparedness, surveillance, research, sanitation, vaccination, responsible antimicrobial use, and rapid response can reduce the impact of future emerging diseases.