Zoonotic Diseases 101: How Viruses Jump From Animals to Humans

Though we don’t know exactly where the novel coronavirus originated, many scientists agree that it probably came from an animal. If that’s true, it means that COVID-19 is a zoonotic disease, or zoonosis—an illness caused by a pathogen that jumped from animals to humans.

The term zoonotic disease might not come up in regular conversation very often, but you surely know quite a few of them by name. Rabies, Lyme disease, AIDS, and plague are all known zoonoses, and scientists believe that Ebola virus disease and severe acute respiratory syndrome (SARS) originated from animal viruses, too. But that list just scratches the surface.

A 2017 study by researchers at the Centers for Disease Control and Prevention (CDC) estimated that 60 percent of the world’s known infectious diseases and up to 75 percent of new or emerging ones are zoonotic, and about 2.5 billion people are affected by a zoonotic illness each year (though only 2.7 million of those cases result in death).

Species and Spillovers

But for every pathogen that manages to escape one type of host and infect another (which is called a spillover event), there are countless others that can’t make the jump. One reason is that different species don’t mingle as closely as you may think.

“To the casual observer, it might look like wildlife in native habitats are all mixed together and coming into close contact with one another, but in fact each species is compartmentalized into a particular ecological niche based on their feeding strategies and environmental requirements,” Bruce Rideout, director of disease investigations at San Diego Zoo Global, tells Mental Floss. “Each of these wildlife species will have an array of parasites or pathogens that have adapted to them, so these pathogens will also tend to be restricted to the ecological niche of the host. As long as ecosystems are intact, those pathogens will tend to stay in their native hosts and not spill over into others.”

According to Rideout, the rise in spillover events in recent decades is partially because humans are disrupting wildlife ecosystems more often. But even if you were to traipse through an undisturbed patch of forest and pet all the animals you see, it’s not a foregone conclusion that you’d fall ill.

For one thing, there’s a chance the pathogens wouldn’t make it into your body in the first place. Epidemiologist and veterinarian Julianne Meisner tells Mental Floss that sometimes “the type of contact needed for transmission isn’t something that would typically happen between an animal and a person.” It’s possible that the animal only transmits a certain virus from mother to offspring, through sexual intercourse, or via an insect that doesn’t bite humans.

The Perfect Storm

But even if one of the animals did have a virus that entered your body, it would still need to infiltrate your cells. To do this, it binds to the receptors on the surface of the cell, which then envelops all or part of the virus. Once inside, the virus hijacks the cell’s systems and uses them to manufacture more virus particles. However, if the virus can’t breach the cell walls in the first place, it can’t survive—and fortunately, many animal pathogens are specialist pathogens, which are only compatible with that species’ cell receptors. Generalist pathogens, on the other hand, are much more versatile.

“The greatest threat to humans is from generalist pathogens that have the ability to infect a wide range of hosts, either because they use cell surface receptors that are conserved across a wide range of species, or because they evolve rapidly and can quickly adapt to a new host,” Rideout says. The avian influenza viruses, for example, can adapt to infecting humans after mutating just once.

As National Geographic reports, there are other factors that impact an animal virus’s ability to cause an outbreak among humans, including how long the virus can survive without a host, how well the virus can thwart a human immune system, and how often humans come into contact with the species that carries the virus. In many cases, that perfect storm never happens, and a virus doesn’t progress beyond its first human host.

But increased human interference in wildlife ecosystems means more opportunities for generalist pathogens to jump to human hosts—and in order to predict which ones could cause the next outbreak or even pandemic, scientists have to first locate as-yet-undiscovered pathogens. Then, they study their behavior to identify those with the capacity to create that perfect storm. While there are various organizations that do this type of research at local, national, and international levels—the U.S. Agency for International Development’s PREDICT arm of the Emerging Pandemic Threats program, for example—the current pandemic has underscored the need for a greater global collaboration on this front.

It's a Small World

In a commentary published in the July 2020 issue of Science, the Wildlife Disease Surveillance Focus Group—a Washington University School of Medicine-affiliated coalition of infectious disease experts, ecologists, and other scientists—advocated for a decentralized, global database to store and share all research on animal pathogens.

“In the past, before modern transportation, spillover events would have been local and spread slowly, giving people elsewhere time to respond,” Jennifer A. Philips, co-director of Washington University’s division of infectious diseases and co-author of the article, said in a press release. “But now the world is so small that an event in one place puts the whole world at risk. This is not someone else's problem. It's everyone's problem.”

And preventing the next zoonotic disease pandemic isn’t only about surveillance and research—it’s also fundamentally linked to the preservation of the wildlife ecosystems themselves.

“The key thing for the general public to understand is that the best way to safeguard human health is to also safeguard wildlife and ecosystem health,” Rideout says. “The threat to us is not from wildlife; it is from the destruction of wildlife habitat and ecosystems … We need to shift our attention to long term sustainability.”