SARS-CoV-2, mpox, chikungunya, hantavirus, Ebola… Why are viral outbreaks on the rise?
The viral news of the past few weeks—which has been intense, to say the least—has sadly reminded us that we are regularly confronted with the emergence or reemergence of viruses. Why is this? What factors are currently contributing to the emergence (or reemergence) and spread of these viruses? Here is an overview of the main factors involved.
Yannick Simonin, University of Montpellier
Since the SARS-CoV-2 pandemic, which has affected more than 700 million people worldwide and caused approximately 7 million deaths, viruses have regularly taken center stage. Chikungunya, mpox, hantavirus, Ebola, Crimean-Congo hemorrhagic fever… These names, once known only to specialists, have moved from the pages of specialized journals to the front pages of the media.
Why do viral outbreaks seem to occur more frequently today than in the past? There is no simple answer to this question, because such outbreaks are always the result of multiple factors. For an outbreak to occur, a certain combination of factors must come together.
From global warming to population displacement and intensive livestock farming, the conditions for a viral “perfect storm” are not set in stone. But their sheer variety should prompt us to prepare for future pandemics. Here’s why.
Repeated Emergencies
In 2022, while the COVID-19 pandemic was not yet over, monkeypox—now renamed mpox by the World Health Organization (WHO)—made a dramatic entrance onto the world stage. This viral disease, which was once confined primarily to Central and West Africa, unexpectedly spread beyond its usual range and rapidly spread worldwide.
Initially transmitted to humans by animals, including certain rodents, the mpox virus then spread rapidly from person to person. Between 2022 and 2023, nearly 100,000 cases were confirmed in more than 120 countries. In France, more than 4,000 confirmed cases were reported, mostly in 2022, making the country one of the hardest-hit in Western Europe. This was an entirely unprecedented situation for this virus…
More recently, the year 2025 was marked in France by an unusual spread of mosquito-borne viruses. Chikungunya infections, for example, reached unprecedented levels, with more than 800 locally acquired cases reported (ten times the previous record!).
Our species was not the only one affected by outbreaks of infectious diseases: that same year, animal diseases with significant health and economic impacts—such as bovine nodular dermatosis—also required the implementation of large-scale surveillance and control programs in France, leaving many farmers feeling distraught and confused.
The year 2026 is no exception—far from it… In May, the Andes hantavirus outbreak aboard the MV Hondius, as well as the Ebola outbreak in the Democratic Republic of the Congo, serve as reminders that these threats are more present than ever.
View the timeline of recurring viral outbreaks from 2014 to 2026
Viruses: The Undisputed Masters of Adaptation
Typically used within the limited context of laboratories, the terms “variant,” “strain,” and “mutants” have spread far beyond the circle of specialists. They illustrate the extraordinary ability of viruses to adapt to their environment.
Viruses are indeed capable of evolving in several ways. Their genomes can undergo small changes (known as mutations; these occur as viruses replicate, and if certain mutations confer an advantage on the virus—allowing it, for example, to infect hosts more easily—that virus is favored). They can also exchange pieces of genetic material with one another (such exchanges are called recombinations). Finally, they can also swap much longer segments, corresponding to entire sections of their genetic material (this is referred to as reassortment).
For these reasons, in the living world, viruses are by far the fastest-evolving organisms. On average, they mutate 100 to 10,000 times faster than bacteria, about 1,000 times faster than parasites, and up to 100,000 times faster than our own cells! It’s a deeply unequal battle in the race of evolution, one in which viruses emerge as the clear winners.
Viruses That Are Largely Inconspicuous
Most of the time, viruses circulate unnoticed, often without even causing any symptoms. This silent spread most frequently occurs within animal reservoirs.
The problem arises when these animals—which can be considered “asymptomatic carriers”—come into contact with another species susceptible to these “silent” viruses. The virus can then cause symptoms of varying severity and spread, potentially leading to an epizootic among animals or an epidemic among humans.
It is currently estimated that three-quarters of emerging viruses that cause disease in humans originate in animals. The diseases they cause in humans are referred to as “zoonoses.”
And that is precisely where the heart of the problem lies: this seemingly unusual encounter between an animal that serves as a reservoir for a virus with devastating potential and humans is becoming increasingly common.
The main cause? Human impact on the environment. By profoundly transforming nature, we are altering the conditions under which infectious agents spread on a global scale.
Vulnerable ecosystems
While viruses are the main players in viral outbreaks, we are the ones who write the script.
Deforestation and the fragmentation of ecosystems—such as in the Amazon, Central Africa, and Southeast Asia—destroy animals’ natural habitats, forcing them to move closer to populated areas. This proximity creates conditions that are particularly conducive to the transmission of viruses.
These upheavals create what are known as “hot spots” of transmission, where wild animals, domestic animals, and humans interact more frequently. These interactions facilitate the spread of viruses from one animal species to another—and potentially to humans—by breaking down the natural barriers that previously confined viruses to a given animal species.
The emergence of the dangerous Nipah virus, which causes severe neurological syndromes that can lead to the death of infected individuals, perfectly illustrates this phenomenon. This virus was first reported in Malaysia in 1998. In that country, deforestation brought bats into closer proximity to pig farms and to the humans who worked there. This new situation allowed the virus to cross several successive species barriers, eventually leading to human cases.
Similarly, Ebola outbreaks in Central Africa are closely linked to the disruption of forest ecosystems and increased contact between wildlife—particularly fruit bats, which are believed to be the virus’s reservoirs—and human populations. The recent emergence and spread of the Bundibugyo Ebola virus in the Democratic Republic of the Congo fit into this context. In regions where deforestation, the exploitation of natural resources, and population displacement are profoundly altering ecosystems, the resulting fragmentation of these ecosystems fosters increased interactions between species and raises the likelihood of species barrier crossing.
Changes in biodiversity can also contribute to the emergence of viruses. Indeed, high biodiversity can limit the spread of diseases by “diluting” pathogens among many species that are more or less effective at transmitting them.
Conversely, the loss of species diversity we have observed in recent years may facilitate the transmission of infectious agents by eliminating this dilution effect and thereby promoting the dominance of species that are particularly effective at transmitting these pathogens.
Intensive livestock farming that amplifies viruses
Unfortunately, overcrowding is not unique to humans. The growth of the world’s population has also been accompanied by a significant increase in animal farming. Chickens, pigs, cattle, and even ducks are raised at sometimes very high densities in confined spaces.
For example, global poultry meat production has risen from about 9 million metric tons in 1961 to more than 130 million metric tons today, while more than half of all pigs are now raised in intensive farming systems. These conditions facilitate the rapid transmission and evolution of viruses, turning these farms into veritable breeding grounds for disease.
Pigs, for example, can be infected by multiple influenza viruses at the same time, which facilitates the emergence of new variants. This phenomenon led, in particular, to the emergence of the H1N1 influenza virus in 2009.
Cities That Accelerate Epidemics
More than half of the world's population now lives in urban areas, a figure that continues to rise. Densely populated cities—especially large metropolitan areas—have become hotspots for the spread of viruses.
Crowded conditions on public transportation, in schools, and in workplaces lead to more close contact. In some overcrowded neighborhoods, sanitary conditions further increase these risks.
The COVID-19 pandemic, like dengue outbreaks, has shown just how much large cities can act as catalysts for epidemics.
Take, for example, the emblematic case of New Delhi, the capital of India and a megacity with nearly 30 million residents. The population density, the daily flow of millions of commuters on public transportation, and the inadequacies of the public health infrastructure create an ideal environment for the spread of viruses. As a result, the city is regularly hit by dengue outbreaks. The virus circulates so widely there that, according to some estimates, nearly 40% to 50% of the population—and even more in certain neighborhoods—have been infected with the dengue virus at least once!
During the COVID-19 pandemic, Mumbai, India’s economic capital, and New Delhi were also among the hardest-hit areas in the country, illustrating once again how urban concentration amplifies the speed and scale of the spread of the virus.
High-Risk Human Practices
Certain human activities create direct links between species. So-called “subsistence” hunting, which is still practiced in many regions of Africa and Asia, the wildlife trade, and various cultural practices expose humans to unknown viruses.
Live animal markets, where different species are crammed together under often poor sanitary conditions, are high-risk sites for viral emergence. Although the exact sequence of events leading to the emergence of the SARS-CoV-2 coronavirus—which caused the COVID-19 pandemic—has not yet been fully elucidated, the now-infamous live animal market in Wuhan is suspected of having played a central role…
Added to this are certain cultural and religious practices, such as festivals or ceremonies during which live animals are sacrificed and handled, creating potential pathways for viral transmission. Ebola outbreaks have also highlighted the role of certain traditional funeral rites—which involve close contact with the bodies of those who have died from the disease—in amplifying the spread of the virus within communities.
Armed conflicts create conditions conducive to the spread of viruses
Viruses also exploit the deep divisions within our societies. Armed conflicts, mass population displacements, and humanitarian crises create conditions that are particularly conducive to the spread of infectious agents.
In these situations, health systems deteriorate, diagnosis becomes more difficult, vaccination programs are disrupted or even halted, and access to treatment becomes limited or irregular. Furthermore, food insecurity and malnutrition weaken people’s immune systems and facilitate the spread of disease, while the implementation of infection prevention and control measures becomes difficult or even impossible. All of these factors create conditions conducive to the emergence or re-emergence of epidemics that were once considered under control.
The current spread of the Ebola virus in Ituri, in the eastern part of the Democratic Republic of the Congo, clearly illustrates this dynamic. This region, marked by chronic insecurity linked to armed conflict, repeated population displacements, and limited access to health infrastructure, provides particularly favorable conditions for the persistence and spread of the virus.
Globalization Accelerates the Spread of Epidemics
In the past, epidemics took months or even years to spread, limited by the pace of land and sea travel. Today, the speed at which epidemics spread has become staggering—a virus can travel across the globe in less than 24 hours!
The main reason for this rapid spread of viruses lies in the increase in air travel in recent years, which facilitates the movement of goods, animals, and people. Added to this today are other factors related to mobility, notably the massive expansion of high-speed rail networks.
For example, the rapid expansion of China’s high-speed rail network has profoundly altered domestic travel patterns, facilitating massive population movements. This is particularly true during seasonal travel, especially in connection with major traditional holidays, such as the Chinese New Year, which result in massive population movements across the country.
The COVID-19 pandemic has highlighted this unprecedented acceleration, as the virus spread across several continents in just 6 to 8 weeks. In less than 12 weeks, the WHO declared a global pandemic. We know what happened next: the virus spread to more than 180 countries…
Climate Change: The Tipping Point
While all of the factors mentioned above play an important role, the most powerful driver of epidemics is likely climate change. Largely linked to human activities, it acts as a powerful driver of the emergence and re-emergence of viral diseases.
This phenomenon is not limited solely to the steady rise in temperatures we have observed in recent years; it also encompasses the intensification of extreme events (droughts, floods, heat waves, storms, etc.), as well as major disruptions to ecosystems.
Droughts, for example, drive many animals closer to populated areas in search of water and food, while heavy rains and floods encourage the proliferation of rodents by increasing the available resources and driving them toward populated areas, which in particular increases the risk of transmission of viruses such as hantaviruses.
Similarly, rising temperatures and increased heavy rainfall in France have fueled the spread of tiger mosquitoes, contributing to a record outbreak of chikungunya in 2026, including in areas where it had previously been rarely detected.
In conclusion, a viral outbreak is a complex phenomenon that never depends on a single factor, but rather on the convergence of multiple elements: a virus capable of spreading effectively that comes into contact with humans, a vulnerable population, fragile health care systems, high levels of human mobility, favorable climatic conditions…
It is this explosive combination—rather than the characteristics of the virus itself—that transforms a local infection into an epidemic or a pandemic. Thus, the emergence of a virus is the result of a complex balance that is very difficult to predict.
What is certain, however, is that current conditions are particularly conducive to the dawn of the era of emerging viruses. In recent years, we have likely seen only the beginning of this trend…
Yannick Simonin, virologist specializing in the surveillance and study of emerging viral diseases. University Professor, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.