In the United States, for example, some species of mice harbor hantavirus, which periodically infects people, most often when they inhale aerosolized mouse droppings — say, when sweeping out a dusty cabin or garage. Because the infection, which starts like a flu but is fatal in 38 percent of cases, doesn’t spread from person to person (yet), the pandemic risk is currently low, Racaniello says. “A good question is, what would have to happen for that virus to become human-to-human transmissible? And also, what else do mice have that might be a threat to people? But the mice of the United States have barely been sampled, in terms of the viruses they carry.”

During the Obama administration, a U.S.A.I.D. program called PREDICT was created to fill that gap, by using biological surveillance and predictive modeling to identify the most likely sources of zoonotic disease. During the 10 years the program existed, researchers found more than a thousand new potential zoonotic viruses, including an unknown Ebola strain. (Daszak, whose group received financial support from PREDICT, called the project “visionary.”) After the program’s funding ended in September, shortly before the coronavirus outbreak began, the Trump administration authorized two successive six-month extensions. A U.S.A.I.D. spokesperson said that in September, there will be “a planned transition” to a new prevention program, Stop Spillover, with a proposed budget of between $50 million and $100 million over five years. “For these sorts of programs to work, you have to be patient,” Racaniello told me. “But these projects also cost money, and they don’t necessarily seem like they’re producing much in the short term, so they’re the easiest things to cut when you want to cut a budget.”

One challenge for pandemic hunters is understanding which animals are most likely to be the source of viruses. Bats, the original carriers for many zoonotic viruses, rarely pass those diseases to humans directly. (One study found that bats in China harbor more than 500 different coronaviruses, but they also carry paramyxoviruses, influenza and hemorrhagic viruses like Ebola.) More often, Daszak explained, bats infect another animal, which then infects us. “About a fifth of all mammals are bats,” Daszak points out. “And they’re all over the globe. We just don’t realize that, because they fly at night. But they’re out there, pooping all over the place — just like deer and birds, except we don’t see it.” (It’s worth noting that, of the thousands of bat species, only a few — such as the fruit bat and horseshoe bat — are currently thought to be the major reservoirs of zoonotic disease.)

Bats also fly, can live for a long time and thrive across a huge range of habitats, which means that we, and other animals, are more likely to come in contact with them than with other species. Racaniello pointed to an outbreak in Australia in the 1990s that was caused when bats began frequenting a racehorse stable, infecting the horses, which then passed the disease on to their human trainers. In Malaysia, Nipah virus emerged from pigs, on farms in an area that harbored fruit bats. In the Middle East, the MERS coronavirus — which most likely originated in a bat — became endemic in camels, who at some point started passing it on to people.

“Before that outbreak, it wouldn’t have occurred to anyone to look in camels for a pandemic virus,” Racaniello said. “The same is true for a lot of things. For instance, we knew that bats carried SARS-like coronaviruses, but it was only when they started looking for the cause of the first SARS outbreak that they found it had jumped from bats to civet cats, which is how we got it. But as to all the other animals in the world, we pretty much have no idea! So, I think you just need to cast a very wide net.”

To do that, Daszak helped found an ambitious project called the Global Virome Project, which seeks to identify 70 percent of the estimated 1.6 million potentially zoonotic viruses over 10 years, at a cost of $1.2 billion. “We found the closest relative to the current SARS-CoV-2 in a bat in China in 2013,” Daszak told me. “We sequenced a bit of the genome, and then it went in the freezer; because it didn’t look like SARS, we thought it was at a lower risk of emerging. With the Virome project, we could have sequenced the whole genome, discovered that it binds to human cells and upgraded the risk. And maybe then when we were designing vaccines for SARS, those could have targeted this one too, and we would have had something in the freezer ready to go if it emerged.”