What sparked the COVID-19 pandemic remains a mystery, according to the Council on Foreign Relations. Several hypotheses exist, but each lacks sufficient evidence to explain the cause of the crisis. Many scientists believe that SARS-CoV-2, the coronavirus that causes COVID-19, spread from animals to humans—known as zoonotic transmission—in late 2019. But there are several other possibilities: a naturally emerging virus that infected people outside of a laboratory; a naturally emerging virus that was studied inside a lab and leaked; or a virus that was produced as a result of experimentation in a lab.

The World Health Organization has taken a somewhat different stance, stating that it’s highly probable that the virus that causes COVID-19 originated in wild bats that live in caves around Wuhan, China, and may have been passed to a second animal species before infecting humans. Many of the most devastating epidemics of recent decades—including AIDS, avian influenza and Ebola—were triggered by animal viruses that spilled over into people.

While most of us would undoubtedly agree that worldwide pandemics are now one of our greatest concerns, there’s another critical issue that needs our immediate and utmost attention: climate change. What might surprise you is that these two urgent challenges are connected: researchers have recently found that changes in climate can increase infectious disease risk in animals, with the potential to spread to humans.


Scientists have determined that agricultural and domestic animals have given us our most deadly pathogens. For example, measles came to us from cattle, many centuries ago. If not for vaccines, this virus would still be a routine and deadly part of childhood.

Here’s another stunner: the path of deadly viruses isn’t a straight line from animals to people. Recently, an international research team led by scientists at Washington, D.C.’s Georgetown University has found that humans might give viruses back to animals more often than previously thought.

Unfortunately, despite the ever-present danger of a new virus emerging and setting off a worldwide pandemic, there is no global system to screen for wildlife viruses that eventually may jump to humans. Luckily, though, experts and researchers are now coming forward to propose a global, wildlife biosurveillance system to identify and monitor wildlife markets and other hot spots for animal viruses that could cause human disease before the next pandemic emerges.

A rapidly warming planet increases disease risk for wildlife worldwide

As a result of the global pandemic caused by SARS-CoV-2, understanding how the spread, severity and distribution of animal infectious diseases could change in the future has reached a new level of importance.


Middle East respiratory syndrome (MERS) is a viral disease caused by a novel coronavirus. Although most human cases of MERS-CoV infections have been attributed to human-to-human infections in health-care settings, current scientific evidence suggests that dromedary camels are a major reservoir host for MERS-CoV and an animal source of MERS infection in humans.

Working under the “thermal mismatch hypothesis,” which is the idea that the greatest risk for infectious disease in cold-climate-adapted animals—such as polar bears—occurs as temperatures rise, while the risk for animals living in warmer climates occurs as temperatures fall, scientists at the University of Notre Dame, the University of South Florida and the University of Wisconsin-Madison collected data from more than 7,000 surveys of different animal host-parasite systems across all seven continents to provide a diverse representation of animals and their pathogens in both aquatic and terrestrial environments.

Researchers also collected historical precipitation and temperature records at the time and location of each survey and long-term climate data for each location to understand how temperatures affected animal disease risk in different climates and how these patterns varied depending on the traits of the animals and the pathogens.

Next, the scientists coupled their models with global climate-change projections to predict where the risk of animal infectious diseases might change the most.


Earth’s warming climate is encouraging the spread of ranavirus, an infectious disease which has been linked to mass mortality events in various amphibian species. If temperatures continue to rise, scientists expect ranavirus outbreaks to become more severe and to occur more frequently.

In a paper published in the journal Science in November 2020, the researchers stated that on average, animal hosts from cool and warm climates experienced increased disease risk at abnormally warm and cool temperatures, respectively, as predicted by the thermal mismatch hypothesis. So, their analysis suggests that global warming will shift infectious diseases away from the equator, with decreases of animal infectious diseases in the lowland tropics and increases in the highland tropics, temperate and cooler regions of the planet. In other words, the predicted increases in infectious diseases at cooler locations outweighed the decreases at warmer locations, potentially suggesting a net increase in animal infectious diseases with climate change.

Given that most emerging infectious disease events have a wildlife origin, say the researchers, this is yet another reason to implement mitigation strategies to reduce climate change.

Humans frequently transmit their diseases to wildlife

Infectious diseases, however, don’t only flow from animals to people. In a study published March 22, 2022, in the science journal Ecology Letters, the authors describe nearly 100 different cases where diseases have undergone “spillback” from humans back into wild animals, much like how SARS-CoV-2 has been able to spread in mink farms, in wild white-tailed deer, and in lions and tigers that live in zoos.


Hundreds of white-tailed deer in North America have tested positive for SARS-CoV-2. Scientists want to understand how the virus gets into deer, what happens as it spreads among them, and what risk these infections might pose for other wildlife and for humans. Close to 30 million deer live in the United States—one for every 10 people—and a few million live in Canada.

To help guide conversations and policies surrounding spillback of human pathogens in the future, researchers went digging through the literature to see how the process has manifested in the past. They used data science and machine learning to study the host-virus network, a new field that aims to predict which viruses can infect humans, which animals host them, and where, when and why they might emerge. Those insights could be critical if scientists want to understand how and why humans share their diseases with animals.

In their new study, the scientists reported that almost half of the incidents identified occurred in captive settings such as zoos, where veterinarians keep a close eye on animals’ health and are more likely to notice when a virus makes the jump. Additionally, more than half of the cases they found involved human-to-primate transmission, an expected result both because pathogens find it easier to jump between closely related hosts and because wild populations of endangered great apes are so carefully monitored.

This supports the notion that we tend to detect pathogens in the places where we spend a lot of time and effort looking, with a disproportionate number of studies focusing on charismatic animals that live in zoos or near humans. It brings into question which cross-species transmission events we may be missing and what this might mean not only for public health, but for the conservation and well-being of the species being infected.

We know of about 100 different cases where diseases have “spilled back” from humans into wild animals. Pathogens find it easier to jump between closely related hosts. That’s why wild populations of endangered great apes, such as mountain gorillas, are so carefully monitored. ©Richard de Gouveia

Disease spillback has also recently attracted substantial attention due to the spread of SARS-CoV-2 in wild white-tailed deer in Canada and in the United States. Some data suggest that deer have given the virus back to humans in at least one case, and many scientists have expressed the broader concern that new animal reservoirs might give the virus extra opportunities to evolve new variants.

But the Ecology Letters study also highlights a sliver of good news: scientists can use artificial intelligence to anticipate which species might be at risk of contracting the virus. When the researchers compared species that have been infected with SARS-CoV-2 to predictions made by other researchers earlier in the pandemic, they found that scientists were often able to guess correctly. Sequencing animal genomes and understanding their immune systems has paid off.

Spillover may be predictable, the authors conclude, but the biggest problem is how little we know about wildlife diseases. We’re watching SARS-CoV-2 more closely than any other virus on Earth, so when spillback happens, we can catch it. It’s still much harder to credibly assess risk in other cases where we’re not able to operate with as much information. As a result, it’s hard to measure how severe a risk spillback poses for human health or wildlife conservation, particularly for pathogens other than SARS-CoV-2.


Bats play an immense role in controlling insects, in pollination and as an ancillary for reseeding forests. They also act as reservoirs for viruses. More than 130 types of viruses have been recognized in bats, including about 60 kinds of zoonotic viruses. While the bats may live a peaceful coexistence with them, they are extremely pathogenic in humans.

Long-term monitoring, however, will help us establish baselines for wildlife health and disease prevalence, laying important groundwork for future studies. If we’re watching closely, we can spot cross-species transmission events much faster and act accordingly.

Global wildlife surveillance acts as an early warning for the next pandemic

While it’s impossible to know exactly how often animal viruses spill over into humans, it’s certain that coronaviruses alone have caused outbreaks in people three times in the last 20 years. Even a decade ago, it would have been difficult to conduct worldwide surveillance at the human-wildlife interface. But because of technological advances, it is now feasible and affordable; and it has never been more obvious how necessary it is. A global system of wildlife surveillance would identify viruses in wild animals that have the potential to infect and sicken people before another pandemic begins, argues a diverse group of ecologists, infectious disease experts and wildlife biologists in a perspective article published on July 9, 2022, in the journal Science.

Every animal has its own set of viruses, with some overlap across species. Often, an animal species and its viruses have lived together for so long that they’ve adapted to one another, and the viruses cause either no symptoms or only mild to moderate disease. But when different animal species that don’t normally have much contact are brought together, some viruses are able to jump from one species to another. These viruses can be lethal to a newly infected species with no natural immunity.

In some cases, viruses are as deadly in their wild animal hosts as in us. An example is chimpanzees and simian immunodeficiency virus, which infects a variety of Old World, nonhuman primates and produces a clinical syndrome in rhesus monkeys and other susceptible macaque species. Simian immunodeficiency virus has many important parallels to AIDS. ©Patrick J. Endres

Human activity is making such spillover events more and more likely. As the world’s population continues to grow, the demand for natural resources skyrockets. People push into wild areas to make space for new businesses and homes, and to access resources to fuel their economies and lifestyles. Wild animals are caught and sold for consumption, or as exotic pets at wildlife markets, where diverse species are jumbled together under crowded and unsanitary conditions. Wild-animal parts are shipped around the world as trinkets or as ingredients for traditional or alternative medicines. And yet, there is no international system to screen for disease-causing viruses associated with the movement of wildlife or wildlife products.

The authors of the July 2022 Science paper suggest the establishment of a global surveillance network to screen wild animals and their products at hot spots, such as wildlife markets. The idea would be to have local teams of researchers and technicians extract viral genomes from animal samples, rapidly sequence them on-site and upload the sequences to a central database in the cloud. The cost and size of the necessary scientific equipment has dropped in recent years—there’s now a genetic sequencer available that is the size of a USB stick—making such screening affordable even in resource-limited settings where most such hot spots are located. There are people with the expertise and skills to do this kind of work safely almost everywhere in the world, but they just haven’t got the tools.

Once viral sequences are uploaded, researchers around the world could help analyze them to identify animal viruses that may be a threat to people and to develop a better understanding of the universe of viruses that thrive in different environments. By comparing genomic sequence data, researchers could identify what family an unknown virus belongs to, how closely it is related to any disease-causing viruses and whether it has the ability to cause illness in people.


More and more, we are encroaching on wild areas to build cities and homes. This means that we are increasingly living and working near wild animals and insects, which may carry new diseases, upping the chances for a deadly outbreak that could possibly spread around the world. A clue to stopping such a scenario is knowing how viruses adapt from animals to humans.

Biosurveillance beginnings and climate change caps

In the past, before modern transportation, spillover events would have been local and would have spread slowly, giving people elsewhere time to respond. But now the world is so small that an event in one place puts the whole planet at risk.

And, in bygone days, a rapidly warming world, irreversible climate change and zoonotic diseases would have seemed to be facets of science fiction; the makings for a tale of an apocalypse. But with people building developments and homes in previously wild areas—living and working near unknown animals and insects, which can carry unknown diseases—the chances for more deadly outbreaks that could spread around the globe are increasing.

I’d say that capping climate change and beginning biosurveillance may be the twin solutions for our two most top-of-mind threats.

Here’s to finding your true places and natural habitats,