“This is something we have to be very concerned about, and not only because it could establish an alternative reservoir that could then be a source for humans,” says Raina Plowright, a disease ecologist and veterinarian, and associate professor at Montana State University. “In every reservoir, there are going to be different selective pressures on the pathogen, so the virus will evolve in different ways to overcome whichever barriers are present within that species. If we started to have coronavirus circulating in different species, all having slightly different genotypes, then we also have the possibility for new coronaviruses emerging that may be sufficiently different from the current one that they may evade vaccine-induced immunity.”

Those alternative hosts might be bats, the coronaviruses’ apparent original home. Last September, a team of researchers from several institutions estimated that up to 40 species of North American bats might be susceptible to infection and could serve as viral reservoirs. It also might mean nonhuman primates: Johnson, whose NIH-funded project works in South America, worries about possible viral traffic between humans and forest-dwelling monkeys.

But it also could mean species so small that we don’t take notice of them, even though they already live among us and bring diseases near us. Last summer, a team of Canadian researchers showed in lab experiments that North American deer mice—which live everywhere from forests to suburbs and play a role in transmitting Lyme disease and hantavirus—can become infected with SARS-CoV-2, harbor it without symptoms, and pass it to other mice. Whether this would translate to mice in the wild is unknown.

Pretty much everyone who is thinking about the problem of spillback—and the possibility of what scientists are coming to call “secondary spillover” back into the human world—calls for more funding to create some sort of monitoring system: on animal farms, among farm workers, of free-living wildlife. There is a model for how that might work, an existing surveillance system that keeps track of one persistent and intermittently deadly disease: the flu.

Influenza’s ancestral terrain is wild water birds, which pick up the virus, carry it with them as they migrate across the globe, and poop it down onto human society—including onto farms, where it can find fresh hosts, adapt, and produce strains of swine and avian flu that find their way to humans. Because of that persistent risk, an elaborate surveillance network has been built up, which includes scientists from the World Health Organization, the health agencies of various nations, and academic research groups. They sample viruses in waterfowl, monitor pathogens in wild birds and poultry, and track the evolution of the seasonal flu strains that infect humans, looking for the emergence of something threatening and new.

That system didn’t come out of nowhere, though. It didn’t even, entirely, come out of the perception that influenza is a profound public health burden. In its journey from the wild to the human world, the flu passes through an industry that is eager to avoid outbreaks in domesticated animals—like the 2015 avian flu epidemic that devastated the Midwestern turkey industry—that are enormously costly to quell. “The entry point for influenza is agriculture,” says Colin J. Carlson, a global change biologist, assistant professor at Georgetown University Medical Center, and principal investigator for a consortium called the Viral Emergence Research Initiative. “The reason that we’re able to have the global preparedness system that we have is because we know the spillover interface. It’s one that we regularly monitor, and it’s one that has the financial and organizational resources to do that.”