Emerging infectious diseases are infections that are newly recognized, are increasing in incidence or geographic range, or are caused by organisms that have developed important new characteristics. They include diseases that appear for the first time in human populations as well as familiar infections that suddenly spread in new places or become harder to control.
The appearance of a new infectious threat can seem unpredictable, but it usually reflects identifiable changes in the relationship among microbes, people, animals, and the environment. A virus may acquire the ability to infect humans. A pathogen already circulating in people may evolve resistance to treatment. A mosquito-borne infection may expand into a region where the climate and ecological conditions have become more favorable. Or changes in human behavior and movement may give an existing pathogen opportunities to spread that it did not previously have.
Understanding those mechanisms matters because emergence is not simply a matter of discovering dangerous microbes. It is a process shaped by biology, ecology, technology, and human society.
What makes an infectious disease “emerging”?
An infectious disease can be considered emerging when its occurrence or public-health significance changes substantially. The term can therefore apply to several different situations.
A previously unknown pathogen may begin infecting humans. Another infection may have existed in people for years but suddenly appear in a new population or geographic area. A known disease can also become more common because conditions have changed in ways that favor transmission.
Some emerging diseases are zoonotic, meaning they are infections that can pass between animals and humans. Many pathogens that infect humans originated in other animals, although most animal microbes never become successful human pathogens.
Emergence should also be distinguished from re-emergence. Re-emerging diseases are infections that were previously declining or under control but return or increase substantially. The underlying causes can overlap with those of newly emerging diseases, including changes in immunity, pathogen evolution, human behavior, and environmental conditions.
Most new threats begin with an ecological opportunity
A pathogen does not become a major human disease merely because it exists. It needs a way to reach susceptible people, infect them successfully, reproduce, and spread onward.
For a pathogen that normally circulates in animals, this often means overcoming several barriers. It must encounter humans, gain access to human tissues, survive well enough to establish an infection, and—if sustained human transmission is required—be capable of passing from one person to another.
Human exposure to animals creates many opportunities for these encounters. Farming, hunting, wildlife trade, animal handling, and changes in land use can bring people and wildlife into closer contact. Domestic animals can sometimes serve as intermediate hosts between wildlife and humans. Crowded conditions can then amplify transmission once a pathogen has crossed into people.
This is why disease emergence is often less about a single dramatic event than about a chain of opportunities.
Pathogens evolve, and some changes can alter their behavior
Viruses, bacteria, fungi, and parasites evolve as they reproduce. Mutations and, for some organisms, other forms of genetic change can produce variants with different biological properties. Most changes are neutral or disadvantageous, but occasionally a change gives a pathogen an advantage under particular conditions.
For viruses, evolutionary changes can affect characteristics such as how efficiently the virus enters cells, how well it replicates, how readily it transmits, or how it interacts with the immune system. Bacteria and fungi can likewise acquire characteristics that influence their ability to survive, cause disease, or withstand antimicrobial treatment.
Antimicrobial resistance is an important form of pathogen evolution. When bacteria, fungi, parasites, or other microbes become less susceptible to drugs used against them, infections that were previously easier to treat can become more difficult to control. Resistance does not necessarily create a new disease, but it can transform an existing infectious threat.
Evolution also occurs after a pathogen enters a new host population. The resulting changes are not directed toward making a microbe “more dangerous.” They reflect natural selection acting on variation under particular environmental and transmission conditions.
Zoonotic spillover is a critical step
A spillover event occurs when a pathogen moves from an animal host into humans. Spillover is common in the broad sense that humans are repeatedly exposed to animal microbes. What is unusual is a spillover that produces sustained human transmission.
The probability of spillover depends partly on how often humans encounter infected animals and their environments. It also depends on the pathogen itself, including whether it can infect human cells and reproduce effectively.
After a spillover, several outcomes are possible. The pathogen may fail to establish an infection. It may infect a small number of people but stop spreading. It may establish limited human-to-human transmission. In rarer circumstances, it may become capable of sustained transmission and spread widely.
The distinction is important: animal-to-human transmission and a self-sustaining human epidemic are not the same event.
Human movement can turn a local event into a wider problem
Modern transportation allows infected people, animals, and contaminated materials to move long distances in a short time. A pathogen no longer has to spread gradually outward from its point of origin.
Travel alone does not determine whether an infection will spread. The pathogen must still encounter susceptible people and find conditions that support transmission. But extensive movement can connect populations that would otherwise have little contact, making geographic containment more difficult.
Urbanization can have a similar effect within communities. Dense populations provide more opportunities for person-to-person transmission, particularly for pathogens spread through respiratory droplets or aerosols, close physical contact, or contaminated surfaces.
At the same time, cities can have strong health infrastructure, surveillance systems, and medical resources that help detect and control outbreaks. Urbanization therefore changes both the opportunities for transmission and the capacity to respond.
Environmental change can shift where diseases occur
Infectious diseases are closely tied to their environments. Changes in temperature, rainfall, water availability, vegetation, and animal populations can alter the conditions under which pathogens, hosts, and vectors survive.
For vector-borne diseases, these relationships can be especially important. A vector is an organism—often a mosquito, tick, or other arthropod—that transmits a pathogen between hosts. Environmental conditions can influence where vectors live, how abundant they are, how often they feed, and how efficiently certain pathogens develop within them.
Climate-related changes can therefore affect the geographic and seasonal patterns of some infectious diseases. The effects are not uniform, however. Temperature is only one factor among many. Land use, housing, public-health measures, human behavior, local ecology, and the availability of suitable hosts can all determine whether a disease can establish itself in a particular place.
Environmental disruption can also increase contact between humans and wildlife. Clearing forests, expanding agriculture, building roads, and developing previously less-disturbed areas can change animal movement and habitat while increasing human exposure to wildlife and their parasites or pathogens.
Human behavior is part of the biology of disease transmission
Human choices influence which pathogens get opportunities to spread. Changes in food production, animal husbandry, sanitation, housing, sexual behavior, travel, recreational activities, and healthcare practices can all alter transmission.
Food systems are one example. Large-scale production and movement of animals and food can create complex networks through which pathogens may spread. Poor sanitation can allow fecal-oral transmission, while inadequate infection-control practices can facilitate transmission in healthcare settings.
Behavior can also affect exposure to vectors and wildlife. Activities that change where people spend time, what animals they encounter, or how they interact with their environment can alter the probability of infection.
This does not mean that individual behavior is solely responsible for emerging diseases. People operate within larger systems—economic, environmental, agricultural, medical, and social—that shape the choices and exposures available to them.
Weak surveillance can make a disease seem to appear suddenly
Sometimes a disease is not truly new when it first attracts widespread attention. It may have been circulating undetected or misdiagnosed for some time.
Detection depends on whether healthcare workers recognize the illness, whether appropriate diagnostic tests exist, whether samples are collected and analyzed, and whether unusual cases are reported to public-health authorities. Diseases with nonspecific symptoms can be particularly difficult to identify.
Improved surveillance can therefore produce an apparent increase in disease even when part of the change reflects better detection.
Modern laboratory techniques have made it possible to identify pathogens more precisely than in the past. Genetic sequencing can help investigators determine whether apparently separate cases are related, track the evolution of a pathogen, and identify previously unrecognized organisms or transmission patterns.
Why some pathogens spread while others disappear
Crossing into humans is only the beginning. To establish itself, a pathogen must overcome a series of biological and epidemiological obstacles.
Transmission depends on factors such as how much pathogen an infected person sheds, how it leaves the body, how long an infected person remains contagious, how efficiently it enters a new host, and how many susceptible people are available.
Population immunity is another major factor. If many people have immunity from previous infection or vaccination, transmission may be reduced for pathogens against which that immunity is effective. If a population has little or no immunity to a newly introduced pathogen, more people may be susceptible.
Chance also matters. A pathogen may have the biological capacity to spread but encounter too few opportunities to establish sustained transmission. Conversely, a small number of transmission events can sometimes produce a larger outbreak if they occur under favorable conditions.
Emergence is not the same as inevitable pandemic spread
The phrase “emerging infectious disease” can sound synonymous with “next pandemic,” but that is not scientifically justified.
Most infectious threats do not become global pandemics. Some remain confined to particular regions or animal populations. Others cause occasional human infections without sustained transmission. Some spread widely but are limited by existing immunity, public-health interventions, treatment, or characteristics of the pathogen itself.
Risk assessment therefore requires more than asking whether a pathogen is new. Scientists consider how it spreads, which hosts it infects, how severe the resulting disease can be, whether effective treatments or vaccines exist, how much immunity is present, and whether surveillance can detect transmission early.
The combination of characteristics matters more than any single feature.
Why emerging diseases are difficult to predict
Disease emergence is a complex interaction of changing biological and social systems. A pathogen can evolve in ways that are difficult to anticipate, while human behavior, animal populations, land use, and environmental conditions are also changing.
Scientists can identify risk factors and monitor warning signs, but prediction has limits. It is generally easier to recognize conditions that increase the opportunity for emergence than to determine precisely when, where, and which pathogen will produce a major outbreak.
That is why preparedness and surveillance are central to infectious-disease prevention. Monitoring unusual illnesses in people and animals, maintaining laboratory capacity, tracking pathogen evolution, improving infection control, and sharing information can help identify problems before they become much larger.
The same principle applies to antimicrobial resistance: preserving effective treatments depends not only on developing new drugs but also on detecting resistance, using existing antimicrobials appropriately, and limiting opportunities for resistant organisms to spread.
What the public can take from this
Emerging infectious diseases are not random intrusions into an otherwise stable world. They arise when changing conditions create opportunities for pathogens to infect new hosts, spread more efficiently, evade existing defenses, or reach populations that were previously protected.
For the public, that means the most useful way to think about emerging infections is not as an endless succession of mysterious new microbes. The important question is what has changed to make transmission possible now.
Sometimes the answer is pathogen evolution. Sometimes it is a new interaction between people and animals. Sometimes it is environmental change, increased travel, inadequate surveillance, declining immunity, antimicrobial resistance, or several of these factors at once.
Those mechanisms also explain why infectious-disease preparedness is broader than responding to outbreaks after they begin. Detecting unusual infections, understanding how pathogens move through populations, maintaining medical and laboratory capacity, protecting effective treatments, and reducing high-risk exposures can all narrow the opportunities that allow a local biological event to become a larger public-health threat.
