Emerging viruses are pathogens that have recently appeared in a human population for the first time, or that existed before but are rapidly increasing in incidence or geographic range. They appear because of a collision between viral biology and human behavior: viruses mutate constantly, animals harbor vast pools of unknown pathogens, and the ways people reshape landscapes, move around the planet, and interact with wildlife keep creating fresh opportunities for those pathogens to cross into humans. No single factor explains every emergence event, but the recurring ingredients are surprisingly consistent.
Why RNA Viruses Dominate the Emerging Threat List
Most viruses that make headlines are RNA viruses: Ebola, influenza, SARS-CoV-2, Zika, dengue, HIV. That is not a coincidence. RNA viruses replicate with enzymes that lack the error-checking machinery found in DNA-based organisms, so they accumulate mutations at a far higher rate. This genetic sloppiness is actually an advantage: it lets them explore enormous genetic territory in a short time, adapting quickly to new hosts, evading immune defenses, and developing resistance to drugs.1Europe PMC. Adaptive value of high mutation rates of RNA viruses: separating causes from consequences Most of those random mutations are harmful to the virus itself and go nowhere. But in a population of billions of viral particles, even a tiny fraction of beneficial mutations can be enough to open a door into a new species.
This is why RNA viruses account for a disproportionate share of new human infections. They evolve on timescales of weeks and months rather than centuries, which means they can respond in near-real-time to new selection pressures. A virus circulating in bats might acquire a mutation that lets it bind to a human cell receptor, or one circulating in humans might mutate to partially escape vaccine-induced immunity. The SARS-CoV-2 spike protein offers a vivid example: a single mutation called N501Y strengthened the virus’s grip on the human ACE2 receptor through additional molecular interactions, contributing to the higher transmissibility of several variants of concern.2eLife. N501Y mutation of spike protein in SARS-CoV-2 strengthens its binding to receptor ACE2 Another mutation, V367F, increased binding affinity by stabilizing a key structural region of the spike protein.3PubMed Central. V367F Mutation in SARS-CoV-2 Spike RBD Emerging during the Early Transmission Phase Enhances Viral Infectivity through Increased Human ACE2 Receptor Binding Affinity These are the kinds of small, incremental genetic changes that can turn an animal virus into a human pathogen, or a manageable outbreak into a pandemic wave.
Bats and the Mystery of Tolerant Hosts
Bats are the most frequently cited natural reservoir for emerging viruses, linked to Ebola, SARS, MERS, Nipah, and several other pathogens. The reason is not that bats are uniquely “dirty” or dangerous, but that their immune systems appear to work differently from those of most other mammals. When experimentally infected with high doses of Ebola virus and MERS-CoV, bats showed minimal or no clinical disease even as viral levels in their blood and lung tissue reached extremely high concentrations.4Nature. Lessons from the host defences of bats, a unique viral reservoir In other words, bats can carry enormous viral loads without getting sick. This tolerance, shaped over millions of years of coevolution, means viruses can persist and diversify inside bat populations without killing their hosts.
That said, the framing of bats as uniquely dangerous reservoirs is being reconsidered. A growing body of research argues that bats operate a highly efficient anti-inflammatory immune response, and that with the exception of a handful of viruses, they pose relatively little direct zoonotic danger to humans.5PubMed Central. Revising the paradigm: Are bats really pathogen reservoirs or do they possess an efficient immune system? The real danger is not bats themselves, but the situations in which humans come into contact with bats or the animals bats infect: deforestation pushing bat colonies closer to farms, livestock serving as intermediate hosts, or live-animal markets mixing species that would never naturally encounter one another.
How Spillover Actually Works
The moment a virus jumps from an animal to a human is called a spillover event. It sounds like a single dramatic leap, but it usually involves a chain of conditions lining up. At minimum, there needs to be a source host (the species that naturally carries the virus), a recipient host (in this case, a human), and often a bridge or intermediate host that facilitates the transfer. That intermediate host can be another vertebrate or an insect vector like a mosquito.6PubMed Central. Zoonotic spillover: Understanding basic aspects for better prevention
Intermediate hosts can also serve as mixing vessels. When a single animal is co-infected by two different viral strains, their genetic material can recombine or reassort, producing a hybrid virus with properties neither parent had. Pigs, for example, can be infected by both avian and human influenza strains, and that mixing is one of the scenarios virologists watch closely for pandemic potential. The spillover itself may happen through direct contact with an animal’s blood or bodily fluids, through contaminated food, through an insect bite, or even through aerosolized material in a cave or barn. Each pathway creates a different kind of risk, and each requires a different kind of intervention.
Deforestation and Land Use Change
Human reshaping of the natural landscape is one of the strongest drivers of viral emergence. Deforestation, urbanization, and agricultural expansion push people and livestock into habitats where wildlife previously lived undisturbed. These changes accelerate the spread of zoonotic pathogens and increase the chance of transmission.7PubMed Central. Emerging zoonotic diseases originating in mammals: a systematic review of effects of anthropogenic land-use change When you cut into a tropical forest to build roads or plant crops, you create long edges where human activity meets wildlife habitat. Animals that thrive at those edges, like rodents and certain bat species, tend to be exactly the ones most likely to carry zoonotic viruses.
Reducing habitat fragmentation and minimizing the perimeter of these human-wildlife edges has been flagged as one of the most straightforward landscape-level strategies for lowering spillover risk.8The Lancet Planetary Health. Land use-induced spillover: a call to interdisciplinary research to prevent zoonotic disease emergence In practice, though, these edges keep growing. Human population expansion and rising demand for land and resources mean the conditions that favor viral emergence are getting more common, not less.
Industrial Farming and Crowded Livestock
Agricultural intensification creates its own set of risks. Concentrating large numbers of genetically similar animals in confined spaces gives viruses ideal conditions to spread rapidly, mutate, and jump to new hosts. Practices associated with intensive farming, including sustained antibiotic use, dense animal confinement, and long-distance movement of livestock, all contribute to zoonotic disease emergence.9PubMed Central. Understanding and managing zoonotic risk in the new livestock industries The highly pathogenic avian influenza outbreaks of recent years illustrate the pattern: viruses circulating in wild birds enter poultry operations, amplify in densely packed flocks, and occasionally acquire mutations that allow infection of mammals, including humans.
The tension here is real and hard to resolve. Intensification makes animal products cheaper and more accessible to growing populations. But the same efficiency that drives down cost also drives up biological risk by creating enormous, genetically uniform viral incubators.
Climate Change and Mosquito-Borne Viruses
Rising temperatures and shifting rainfall patterns are expanding the geographic range of the mosquitoes that carry viruses like dengue, Zika, chikungunya, and yellow fever. Warmer winters and altered precipitation have allowed key vector species to establish themselves in parts of Europe and other regions where they were previously absent.10PubMed Central. Vectors on the Move: How Climate Change Fuels the Spread of Arboviruses in Europe Modeling studies project that as optimal temperature ranges for transmission shift poleward, populations in temperate regions that have never dealt with these diseases will increasingly fall within the transmission zone.11PubMed Central. Global expansion and redistribution of Aedes-borne virus transmission risk with climate change
This does not mean malaria or dengue will necessarily become common in, say, northern Europe tomorrow. Local infrastructure, public health capacity, and housing quality all modulate actual disease burden. But the biological preconditions for transmission are spreading, and that changes the calculus for countries that historically considered these diseases someone else’s problem.
Wildlife Trade as a Mixing Ground
The global wildlife trade moves enormous numbers of live animals across borders, creating exactly the kind of close, sustained cross-species contact that favors pathogen exchange. An analysis of 40 years of trade data found that traded mammals are about one-and-a-half times as likely to share pathogens with humans compared to non-traded mammals. Illegal trade and live-animal trade raised the risk further. On average, a wildlife species shared an additional pathogen with humans for every decade it was traded.12PubMed. Wildlife trade drives animal-to-human pathogen transmission over 40 years
The composition of the risk is also striking. Roughly a quarter of mammal species currently in wildlife trade harbor about three-quarters of known zoonotic viruses, at levels much higher than either domesticated or non-traded wild mammals.13PubMed. Mammals, wildlife trade, and the next global pandemic These traded species also carry distinct viral communities compared to livestock, meaning the wildlife trade introduces viruses that humans and their domestic animals have no prior exposure to. Markets where live wild animals are kept in close quarters with other species and with people are particularly high-risk environments for spillover.
Air Travel and Global Connectivity
Even fifty years ago, a new virus emerging in a remote region might burn through a local population and fade out before reaching a major city. That is no longer the case. Air travel can connect any two points on the planet within hours, and this connectivity has increased both the frequency and the geographic reach of infectious disease epidemics.14PubMed Central. Human Mobility and the Global Spread of Infectious Diseases: A Focus on Air Travel A person infected but not yet symptomatic can board a plane in one continent and arrive in another before they know they are sick. The 2003 SARS outbreak, the 2014 Ebola epidemic’s export cases, and the global spread of SARS-CoV-2 all followed international travel routes with remarkable fidelity.
Urbanization compounds this. More than half the world’s population now lives in cities, many of them densely packed, with high volumes of international travel. Dense populations mean more contacts per infected person, faster local spread, and greater likelihood that a traveler will carry the virus elsewhere before containment is possible.
Reassortment and the Influenza Problem
Influenza deserves its own discussion because it has a trick that most other viruses lack. Its genome is split into separate segments, and when two different flu strains infect the same cell, those segments can be shuffled and combined into new arrangements. This process, called reassortment, can produce entirely new viral strains in a single replication cycle.15PubMed Central. Implications of segment mismatch for influenza A virus evolution It is the mechanism behind “antigenic shift,” the sudden appearance of a flu strain so different from its predecessors that the population has essentially no preexisting immunity.
Reassortment is not random chance in the way people sometimes imagine. Experimental co-infections show wide variation in reassortment frequency depending on which strains are involved, ranging from under 5% to over 90%.16PLoS Pathogens. Influenza A virus reassortment is strain dependent Most reassortant viruses are less fit than their parents and quickly disappear. But the sheer volume of influenza infections worldwide means the rare combinations that do work can emerge and spread. Every pandemic flu strain of the twentieth century, including the 1918, 1957, and 1968 pandemics, involved reassortment events. Segmented genomes are found in other virus families too, and the basic principle, that co-infection can generate dramatically new viral progeny, applies more broadly.17PubMed Central. Reassortment in segmented RNA viruses: mechanisms and outcomes
Waning Immunity and the Return of “Conquered” Viruses
Not all emerging viruses are new. Some are old acquaintances returning because population immunity has eroded. Measles and mumps, both considered well-controlled through vaccination, have been re-emerging in countries with sustained high vaccine coverage. For mumps in particular, waning vaccine-induced immunity has been identified as a major factor behind recent outbreaks.18PubMed. Waning immunity and re-emergence of measles and mumps in the vaccine era
Mathematical modeling has explored what happens as populations gradually shift from immunity acquired through natural infection, which tends to be long-lasting, to immunity acquired through vaccination, which can wane over time. With current estimates of how long vaccine-derived protection lasts, measles would not re-emerge quickly. But the models show that re-emergence is possible several decades after introduction of high vaccination levels, particularly if booster coverage is uneven.19PubMed. Modelling measles re-emergence as a result of waning of immunity in vaccinated populations This creates an underappreciated risk: a generation that grew up with high vaccination rates and almost no natural exposure could become the generation in which previously controlled diseases make a comeback.
Evolution Inside Immunocompromised Patients
One source of new variants that received widespread attention during the COVID-19 pandemic is prolonged infection in people with weakened immune systems. When a virus persists in a single patient for weeks or months because their immune system cannot fully clear it, the virus has an unusually long window to accumulate mutations. There is significant evidence that viral evolution accelerates in immunocompromised hosts, with mutations piling up in the very sites that antibodies target.20PubMed Central. Virus Evolution in Prolonged Infections of Immunocompromised Individuals The Omicron variant of SARS-CoV-2, with its unusually large number of spike mutations, is widely suspected to have evolved during such a prolonged infection, though definitive proof is difficult to establish.
This pathway matters beyond any single pandemic. As populations age and the number of people living with immune-suppressing conditions or medications grows, the frequency of prolonged viral infections may increase, creating more opportunities for accelerated evolution. It is a reminder that emergence is not only about viruses crossing from animals to humans; it can also happen within the human population itself.
How Emerging Viruses Evade Our First Line of Defense
When a virus first enters the body, the innate immune system, particularly the interferon response, mounts the initial defense. This is a generic, fast-acting response that does not depend on having seen the virus before. Emerging RNA viruses have developed multiple strategies to suppress or evade this system, buying themselves time to replicate before the body’s adaptive immunity kicks in.21PubMed Central. Viral Innate Immune Evasion and the Pathogenesis of Emerging RNA Virus Infections Some viruses produce proteins that directly block interferon signaling. Others hide their genetic material inside protective structures that prevent cellular sensors from detecting them. Still others actively degrade the host’s antiviral proteins.
The severity of many emerging virus infections is closely tied to how effectively the virus disables this early immune response. Ebola, for instance, is catastrophically lethal in part because it aggressively suppresses interferon production, allowing the virus to spread unchecked through the body before the immune system can mount a coordinated defense. Understanding these evasion mechanisms is not just academic; it shapes the development of antiviral drugs and helps explain why some people become severely ill while others clear the same virus quickly.
Wastewater Surveillance and the Hunt for Early Signals
One of the most promising developments in detecting emerging viruses before they become epidemics is wastewater surveillance. Metagenomic sequencing of sewage can, in principle, detect any known or novel pathogen circulating in a population, because people shed viral genetic material in their feces and urine before symptoms appear and regardless of whether they seek medical care. A statistical modeling study found that using targeted enrichment panels dramatically improved detection sensitivity and reduced costs, bringing the price of a system capable of catching a SARS-CoV-2-like pathogen early down from millions of dollars per year to a few hundred thousand.22PubMed. Inferring the sensitivity of wastewater metagenomic sequencing for early detection of viruses: a statistical modelling study
The technology is not yet plug-and-play. Untargeted shotgun sequencing of raw wastewater samples has limited sensitivity for detecting human viruses amid the enormous background of bacterial and environmental genetic material. But hybrid-capture approaches that selectively enrich for respiratory viruses have shown strong potential for simultaneously tracking multiple pathogens from a single sample.23PubMed Central. Comparison of metagenomic and targeted methods for sequencing human pathogenic viruses from wastewater Wastewater monitoring is already being used in dozens of countries for SARS-CoV-2 tracking, and the infrastructure built during the pandemic is being repurposed for broader pathogen surveillance.
Predicting Where the Next Spillover Will Happen
Researchers are increasingly trying to move from reacting to outbreaks to predicting where they are most likely to occur. One approach estimates spillover risk by mapping the overlap between areas of high zoonotic virus diversity and high human population density, applying the basic epidemiological principle that transmission increases when susceptible hosts and infectious agents are in close proximity.24One Health. Prediction of viral spillover risk based on the mass action principle Geospatial modeling has become a central tool in this effort, drawing on satellite imagery, climate data, and disease records to identify hotspots. However, a systematic review of these models found that full integration of human health, animal health, and environmental data, the approach known as One Health, was present in only about 15% of published studies.25PubMed Central. Geospatial modelling for zoonotic disease hotspot identification within a One Health framework: a systematic review Most studies leaned heavily on climate variables while underusing data on animal populations and socioeconomic conditions.
Prediction remains difficult for a simple reason: emergence events are rare, high-consequence, and dependent on the alignment of many factors at once. A model can identify high-risk zones, but pinpointing when and which virus will jump is beyond current capability. The practical value lies in directing surveillance resources to the places where they are most likely to catch something early.
Ancient Viruses Written Into Human DNA
The relationship between humans and viruses is far older than any modern emergence event. Roughly 8% of the human genome consists of sequences inherited from ancient retroviruses that infected our ancestors millions of years ago. These human endogenous retroviruses, or HERVs, were once dismissed as inert genetic fossils, but they are now known to play active roles in gene regulation, immune function, and even the formation of the placenta.26PubMed Central. Human Endogenous Retroviruses and Diseases Their structural components, including regulatory sequences and conserved viral genes, continue to influence how human cells behave.
Studying these genomic fossils also provides a window into ancient viral emergence. By comparing endogenous retroviruses across different species, researchers can reconstruct cross-species transmission events that happened deep in evolutionary time, identifying when and how viruses jumped between animal lineages millions of years before humans existed.27PLOS Genetics. Cross-species transmission of an ancient endogenous retrovirus and convergent co-option of its envelope gene in two mammalian orders These ancient records suggest that viral spillover is not a modern aberration. It has been a constant feature of mammalian evolution. What is modern is the pace at which humans are creating new opportunities for it to happen.