Pathogens that cause disease in people and other animals — called zoonotic diseases — aren’t uncommon: Rabies, flu viruses and HIV, the virus that causes AIDS, are all familiar examples. But the threat of such pathogens is growing. Researchers who monitor for emerging infectious diseases typically recognized about three per year, says epidemiologist and disease ecologist Jonna Mazet. In the last decade that’s increased to about five per year.

That uptick is not the fault of the animals that carry these pathogens, says Mazet, of the University of California, Davis. “It’s our human behavior and the risky things that we do that put ourselves at risk for exposure to these viruses.”

Mazet details some of the factors that increase that risk, which include changes in how and where people interact with wildlife. As people push further and further into places that were previously uninhabited — such as mining for materials that make our cell phones lighter — the probability of encountering unfamiliar viruses increases. “We are exposing ourselves to things that just aren’t in our evolutionary history and we’re really at risk,” Mazet says.

The virus behind the pandemic, SARS CoV-2, was one such “jumper.” Evidence suggests that its original evolutionary host was a horseshoe bat, one of many viruses that bats appear to tolerate with little to no ill effects. And there will likely be more such “spillover” events to come.

“We know that in the viral families that are known to cause human disease and be shared with other animals, that there are at least 500,000 viruses still to discover that can infect us and could make us sick,” Mazet says. “We've only scratched the surface.”

Ultimately, we have to rethink how we interact with the environment, Mazet says. “We’re talking about the interconnected health of the whole planet — the people, the animals, the environment — and when we do protect that we protect ourselves for the longer term.”


This video is part of Reset: The Science of Crisis & Recovery, an ongoing series exploring how the world is navigating the coronavirus pandemic, its consequences and the way forward. Reset is supported by a grant from the Alfred P. Sloan Foundation. Watch more videos from Knowable Magazine.