Right now, at least 10,000 viruses capable of infecting humans are lurking in the wild, most of them circulating undetected in animals. Typically, these diseases don’t end up reaching us. But in some cases, a virus will leap from one species to another, a phenomenon known as a “viral spillover.” When emerging infectious diseases hop between animal hosts, and, eventually, to humans, the consequences can be catastrophic.
Viral spillover is the culprit behind some of the most damaging infections in history: In late 2002, the coronavirus that causes SARS, or severe acute respiratory syndrome, jumped from bats to people through an intermediate host, a weasel-like creature called a civet. A decade later, in 2013, an 18-month-old boy from a village in Guinea was playing near a hollow tree that contained bats carrying a mysterious disease — later identified as Ebola. The child became infected and died, possibly setting off an outbreak that killed more than 11,000 people. And while we may never know the exact origins of COVID-19, scientists still believe that the disease that ignited our current pandemic most likely came from a bat, as well.
The potential for such spillover events is likely to increase dramatically as Earth's climate heats up, possibly introducing humans to thousands of new viruses, according to a new study published in Nature last week. That’s because global warming is forcing animals to move to new habitats to find food and cooler temperatures. “That basically shuffles the deck within ecosystems in terms of which species are in contact with each other,” says Colin Carlson, a coauthor on the study and a global change biologist at Georgetown University.
