By carefully tracing the chain of viral transmission, the Italian researchers discovered that asymptomatic people can infect others. That was also the conclusion of the researchers who studied nursing home residents. And U.S. Navy officials involved in the care of infected crew members aboard the aircraft carrier USS Theodore Roosevelt reported that asymptomatic individuals continue to test positive for the coronavirus — and might possibly be able to infect others — for an extended period of 14 days or more. But how often asymptomatic people actually do transmit the virus to others remains uncertain.

The potential to make others sick is not the only risk of being infected without symptoms. In studies from Japan and South Korea, the lungs of asymptomatic people were found to have abnormalities, revealed as hazy regions on their CT scans — the distinctive “ground-glass opacities” that have become a well-known sign of covid-19.

It is not clear what these lung abnormalities might mean for the long-term health of the asymptomatic — a stark reminder that science is only now beginning to understand the multiple ways that this novel pathogen can harm people.