It’s not common to contract Covid-19 from a contaminated surface, scientists say. And fleeting encounters with people outdoors are unlikely to spread the coronavirus.

Instead, the major culprit is close-up, person-to-person interactions for extended periods. Crowded events, poorly ventilated areas and places where people are talking loudly—or singing, in one famous case—maximize the risk…

Health agencies have so far identified respiratory-droplet contact as the major mode of Covid-19 transmission. These large fluid droplets can transfer virus from one person to another if they land on the eyes, nose or mouth. But they tend to fall to the ground or on other surfaces pretty quickly.

Some researchers say the new coronavirus can also be transmitted through aerosols, or minuscule droplets that float in the air longer than large droplets. These aerosols can be directly inhaled.