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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.