Is the Coronavirus in Your Backyard?

There are many theories, none entirely satisfying. An infectious hunter might encounter a deer, Dr. Mubareka noted, but “if they’re good at hunting,” she added, “it’s a terminal event for the deer.”

If an infected hiker “sneezes and the wind is blowing in the right direction, it could cause an unlucky event,” said Dr. Tony Goldberg, a veterinary epidemiologist at the University of Wisconsin-Madison. Or if people feed deer from their porch, they could be sharing more than just food.

And white-tailed deer are expert leapers, reaching heights of eight feet. “If you want to fence deer out of a place, you have to be trying very hard,” said Scott Creel, an ecologist at Montana State University. Deer would have no trouble jumping into alfalfa fields to graze alongside cattle, perhaps inviting a close encounter with a farmer, Dr. Creel said.

Transmission could also happen indirectly, through wastewater or discarded food or other human-generated trash. “Deer, like most other animals, will sniff before they eat,” Dr. Kapur said. And deer release their feces as they feed, creating conditions where other deer might forage in areas contaminated with waste, or snuffle around waste that has feed mixed in, experts say.

But it’s not clear how long the virus would remain viable in a polluted water source or on the surface of a half-eaten apple, or whether enough of it would be present to pose a transmission risk.

An intermediate host, such as an itinerant cat, might ferry the virus from humans to deer. Farmed deer, which have frequent contact with humans, might also pass the virus to their wild counterparts through an escapee or their feces, Dr. Seifert said. (More than 94 percent of the deer in one captive site in Texas carried antibodies for the virus, researchers found — more than double the rate found in free-ranging deer in the state.)