In an interview, Dr. Fauci said, “I don’t think you want to have folks shutting down cities like in northern Italy. We are not at that level. That is a hot spot. Social distancing like in Seattle is the way to go. I’m not talking about locking down anything. There’s a big difference between voluntary social distancing and locking anything down.”

If community spread is being detected now, that means it began, unseen, weeks ago. The greatest concern is for older people, particularly those who have underlying conditions like diabetes, heart disease, lung problems and weakened immunity.

“Don’t go to crowded places, think twice before a long plane trip, and for goodness sake don’t go on any cruises,” Dr. Fauci said.

For people who are particularly vulnerable, he said: “Don’t wait for community spread. Now is the time to do social distancing, whether there is spread in your community or not.”