Some research teams are focusing on the ACE2 receptor, a protein found on the surface of the cells of many species. The coronavirus’s spiky protrusions allow it to bind to these receptors, like a key in a lock, and enter cells.

In 2020, a group of scientists compared the ACE2 receptors of hundreds of vertebrates, mostly mammals, with those of humans to determine which species the virus might infect. (The ACE2 receptors of birds, reptiles, fish and amphibians are not similar enough to ours to raise concern.)

“The predictions have been very good so far,” Harris A. Lewin, a biologist at the University of California, Davis, and an author of the study, said in an email. The scientists predicted, for instance, that white-tailed deer were at high risk for infection.

But some predictions proved entirely wrong: The paper identified farmed mink as a species of “very low” concern — and then in April 2020 the virus raged through mink farms.

Indeed, ACE2 offers only a snapshot of susceptibility. “Viral infection and immunity is much more complex than just a virus binding to a cell,” Kaitlin Sawatzki, a virologist at Tufts University, said in an email.

And of the world’s nearly 6,000 mammalian species, scientists have sequenced the ACE2 receptors of just a few hundred of them, creating a biased data set. These sequenced species include model organisms used in experiments, species that carry other diseases, and charismatic zoo denizens, not necessarily the animals that people are most likely to encounter.

“If a pandemic were to have arisen from a squirrel, we would be like, ‘God, what’s wrong with us? We didn’t even measure the basic biology of a squirrel,’” Dr. Han said.