People with asthma are not more likely to catch COVID-19, but they are more likely to fare poorly if they do. Asthma and similar health conditions cause the lungs to have trouble exchanging air, a situation that viruses such as the flu and the coronavirus exacerbate by filling the lungs with inflammatory cells, Chu said.

There’s no hard cutoff for when a crowd becomes too risky for an asthmatic or elderly person. It’s not that, say, the opera is definitely off-limits but work meetings are guaranteed to be fine. Chu said she recommends that people who have asthma or a lung disease or are otherwise immunocompromised start thinking about telecommuting from work right about now.

For everyone else, decisions depend on what Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security, calls personal “risk preference.” That is, how worried are you about catching COVID-19? If you’re relatively young, healthy, and traveling to a place other than the five restricted countries relatively soon, you might decide that the risk of catching the disease is worth whatever it is you’re doing. Adalja told me that he has not canceled any of his public appearances. Two other experts told me that it’s still too premature for healthy people living in areas without a large number of cases to avoid gatherings or otherwise change their plans.