Why? There’s a theory that youngsters get the four known mild coronaviruses so often that they’re protected.

That’s still a theory. I couldn’t get enough people to agree to put it in the W.H.O. report.

Does that imply that closing schools is pointless?

No. That’s still a question mark. If a disease is dangerous, and you see clusters, you have to close schools. We know that causes problems, because as soon as you send kids home, half your work force has to stay home to take care of them. But you don’t take chances with children.

Are the cases in China really going down?

I know there’s suspicion, but at every testing clinic we went to, people would say, “It’s not like it was three weeks ago.” It peaked at 46,000 people asking for tests a day; when we left, it was 13,000. Hospitals had empty beds.

I didn’t see anything that suggested manipulation of numbers. A rapidly escalating outbreak has plateaued, and come down faster than would have been expected. Back of the envelope, it’s hundreds of thousands of people in China that did not get Covid-19 because of this aggressive response.

Is the virus infecting almost everyone, as you would expect a novel flu to?

No — 75 to 80 percent of all clusters are in families. You get the odd ones in hospitals or restaurants or prisons, but the vast majority are in families. And only 5 to 15 percent of your close contacts develop disease. So they try to isolate you from your relatives as quickly as possible, and find everyone you had contact with in 48 hours before that.