You could tie yourself in knots gaming out the various scenarios that might pose a risk outdoors, but Marr recommends a simple technique. “When I go out now, I imagine that everyone is smoking, and I pick my path to get the least exposure to that smoke,” she told me. If that’s the case, I asked her, is it irrational to hold your breath when another person walks past you and you don’t have enough space to move away? “It’s not irrational; I do that myself,” she said. “I don’t know if it makes a difference, but in theory it could. It’s like when you walk through a cigarette plume.”

Indoors, experts’ opinions start to diverge. Consider, for example, the grocery store—one of the last vestiges of public life. There, Santarpia is far more concerned about touching shared surfaces than breathing shared air, and he makes sure to sanitize his hands before he leaves. Marr said that she tries to go when it’s less crowded, although that’s obviously harder in a big city. Bourouiba’s best advice is to always keep as much distance from other people as possible, and she adds that the onus is on stores to improve their ventilation or limit the number of concurrent customers. Stores must also devise ways of protecting the people at greatest risk: the cashiers and the workers stocking shelves.

Then there are shared spaces like hallways, stairwells, and elevators in apartment buildings. Elevators pose the highest risk, Bourouiba told me, since they’re enclosed boxes with limited airflow. For stairwells and hallways, she advocated a commonsense approach: “If you hear neighbors going out, and there are 10 people in the corridor right now, maybe wait and go later.”