Dr. Han and her colleagues ultimately identified 540 mammalian species that were most likely to host and spread the coronavirus. She had been especially worried that the red fox, which ranked high on her list of at-risk creatures, and is widespread in Europe and North America, would be susceptible to the virus. “We’re just waiting for somebody to report it,” she said.

Just days earlier, in fact, researchers in Colorado had announced that the virus had proved capable of infecting red foxes in the lab. “Oh no!” Dr. Han exclaimed when informed of the finding. “It really sucks to be right in my line of work.”

Last fall, scientists analyzing tissue samples from dead white-tailed deer in Iowa found that the virus was widespread in that species. The discovery intensified concerns that the virus might establish itself in an animal reservoir, mutate and spread to other species, including back to humans. It also opened a rabbit hole: If deer can silently spread the coronavirus, what else could? And what else will?