Spillbacks confound our containment strategies. In theory, we can tame pathogens that prey exclusively on Homo sapiens. We can change our behaviors to make transmission difficult. We can stop drinking waste- ​contaminated water, making the transmission of cholera difficult. We can protect our homes with mosquito screens, making the transmission of malaria difficult. We can eradicate a pathogen altogether, as we did smallpox through a global vaccination campaign. But once a pathogen spills back from humans into wild animals, those options slip away, for we have even less control over the behavior of nonhuman animals than we do over our fellow humans. “Well, now it’s in fish, it’s in frogs, it’s in primates,” the disease ecologist Barbara Han says. “How are you going to get rid of that?”

While the United States has spent millions of dollars surveilling low-income countries overseas for possible spillovers from wild animals into humans, in the United States, disease surveillance in wild species is mostly passive and opportunistic — designed to detect large-scale die-offs of wild animals, not the silent establishment of a pathogen in a new reservoir species. Finding evidence of that requires actively and systematically looking for it. This August, the U.S.D.A. announced a new $300 million program to strengthen disease surveillance in both domestic and wild animals, but until it gets underway in the next couple of years, “the truth is,” the coronavirus expert Linda Saif says, “there is very little funding to study these scenarios where the virus is in humans and might spill back into animals.” It’s likely that we may detect only that subset of coronavirus spillbacks that happen to re-emerge in humans, and in those cases only in the rearview mirror, by piecing together genetic and other clues to reconstruct their prior forays through the bodies of animals.