COVID, monkeypox, Ebola, and SARS all originated in animals. Some researchers think we can predict what’s next, while others believe it’s an impossible task.

By Matthew Hutson

The natural world is a reservoir of plagues. At any moment, untold numbers of viruses circulate among animals. Inevitably, some will cross the species barrier, infecting people and making them sick. Scientists call such an event a “zoonotic spillover.” No one knows how often such spillovers happen—presumably, animal viruses are always establishing footholds that our immune systems destroy. We notice, though, when the viruses propagate. Today, countries around the world are seeing cases of monkeypox, a milder relative of smallpox. Just like covid-19, the disease originated in other animals. It was seen first in monkeys, in 1958, before being detected in a boy, in 1970. Other recent spillovers have caused diseases including Ebola, flu, Lassa, Marburg, mers, Nipah, sars, and Zika.

Dawn Zimmerman, a fifty-one-year-old wildlife veterinarian formerly at the Smithsonian Global Health Program, has spent years studying zoonotic viruses in wildlife in Turkana county, Kenya. On one trip in 2017, she visited an area in the northwest called No Man’s Land. “It’s because no one owns it,” she told me. “People are always fighting over that land.” On a field day, her team might gather early in the morning to drive into the bush, sometimes accompanied by armed guards. They would check rodent traps set the night before, taking oral and rectal swabs from any animal they found, and follow troops of baboons, picking up droppings and sampling them. Occasionally, they would set a trap for a baboon—a cage that closes when a primate pulls on an ear of maize tied to a string—to facilitate sampling. In the evening, they’d use mist nets on riverbanks to catch the bats that emerged after dusk.

Sometimes the team took samples from camels—livestock animals that are known to be “viral reservoirs,” or sources of possible spillover. In one town, a woman named Ester was in charge of the livestock; after having tea in Ester’s house, Zimmerman’s team went out to meet the animals, bringing along medicine for them as a thank-you. They hadn’t brought enough, and an owner pointed what looked like an AK-47 at them. “She just put her finger up, and she’s, like, ‘No!’ ” Zimmerman recalled, of Ester. “And he put his gun away.” To access a different site, they had to cross a river. “The first thing I asked is, ‘Are there crocodiles in this river?’ And they said, ‘No, no, totally hunted out, no problem,’ ” Zimmerman told me. The researchers crossed as part of a large crowd, with Zimmerman immersed to her chest. That night, while they were setting up their bat nets, they saw two pairs of crocodile eyes shining in the water.

While sampling, researchers like Zimmerman wear N95 respirators, rubber boots, one or two pairs of gloves, and Tyvek suits—a getup that can become unbearable in the heat. They lug around a container of liquid nitrogen for storing their samples until they can be frozen and sent to a lab, where researchers will screen them for viruses, then sequence the viruses’ genes to determine if they’re known or novel. In another lab, further analyses might attempt to predict the risk that any novel viruses pose to people. For several years, Zimmerman’s data made its way to PREDICT, a program run by the United States Agency for International Development (U.S.A.I.D.) aimed at predicting, preventing, and containing emerging infectious diseases. From 2009 to 2020, PREDICT’s researchers collected samples from a hundred and sixty thousand animals and people in about thirty countries, and discovered almost a thousand new viruses. It’s since been replaced by deep-vzn (Discovery & Exploration of Emerging Pathogens—Viral Zoonoses), a five-year program, also funded by U.S.A.I.D., which will spend a hundred and twenty-five million dollars to find new viruses in animals around the world. deep-vzn will focus in particular on coronaviruses, filoviruses, and paramyxoviruses—the three viral families that include sars-CoV-2, Ebola, and measles. (U.S.A.I.D. has also launched a hundred-million-dollar effort called stop Spillover, aimed at preventing and catching spillovers, based on knowledge gained from viral surveillance.) “It will be a defining characteristic of this century, these zoonotic spillovers,” Dennis Carroll, the infectious-disease specialist who founded PREDICT, told me. Today, Carroll runs the Global Virome Project (G.V.P.), another successor to PREDICT.

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