Pathogens and Infectious Disease
1. What Is a Pathogen?
Learning outcomes
- I can define a pathogen.
- I can distinguish pathogens from non-pathogenic microorganisms.
- I can identify major categories of pathogens.
- I can explain how pathogens cause disease.
- I can describe the relationship between pathogens and hosts.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Common Mistake: All Bacteria Are Harmful
Most bacteria are not:
pathogens.
Many are harmless, and many perform beneficial ecological or biological functions.
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.
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.
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.
Common Mistake: Infection Always Causes Symptoms
Some infections are:
asymptomatic.
A person may carry and sometimes transmit a pathogen without showing obvious symptoms.
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.
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.
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.
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.