“We’re all struggling with a greater degree of ambient risk than we’re used to,” said Dr. Tim Lahey, an infectious disease specialist and ethicist at the University of Vermont. Each day is an exercise in trying to lower risk: avoid this, scrub that.

Public health practice is as much about reducing risk, as eliminating it — which is often impossible. The AIDS crisis was not stemmed by persuading people to quit sex, Dr. Lahey said. Instead, people adopted tolerable rules like choosing partners carefully and wearing condoms. The term “safer sex” worked because it seemed doable, he added.

The safer-sex equivalent of an outdoor walk, most medical authorities say, is one that involves six feet of distance from others. (Linsey Marr, an engineering professor at Virginia Tech who studies how particles move through air, says she gives it 10 feet just to be cautious.) Governments are beginning to put in place rules to encourage people to spread out. This week, Gov. Andrew M. Cuomo announced a pilot program in New York to close some streets to traffic in order to give pedestrians more space. As of Tuesday, the French must follow new restrictions on outdoor exercise: It can be done alone, for up to an hour a day, within a one-kilometer radius of home. Walkers and runners must carry permission slips that can be checked by authorities.

If the distancing rules are too strict, prohibiting excursions entirely, people could give up, said Dr. Carlos Del Rio, a public health and AIDS specialist at Emory University. “I want to be sure that people don’t get frustrated and say, ‘We won’t be able to defeat this,’ because we can,” he said.

“Our mental health is going to be so important,” added Dr. Spencer, who was treated for Ebola in 2014 and endured 19 days of near-total isolation. “This is only going to get worse.”

“Telling people to stay inside works right now, but in two or three weeks, it’s going to be a tough message to hold up,” he said.

Walks and runs are signs of life to which even doctors and scientists on the front lines are clinging. Amandine Gamble, a postdoctoral fellow at the University of California, Los Angeles, is a co-author of that study that raised alarm last week about how the virus lingers in the air. She walks near her home in Santa Monica every day. She finds complete adherence to the six-foot rule challenging, she said, and if someone crosses her path, she does not panic.