But how could he do that when, because of medical privacy laws, he did not know where the nine infected people lived? “It’s the specificity of the floor you’d like to know,” Mr. Nowicki said. “Like, are the cases on the 7th floor or the 9th floor? Are all the infections on one floor? Or is it spread along all the floors? You’d like to know.”

He is not the only one. Residents call Mr. Nowicki’s wife, Tracy, the city’s director of elder services, demanding to know who in their building is positive, and she gently deters them.

“They want to make sure they don’t knock on their door,” she said. “I totally understand that. I totally understand why the residents that are still healthy want to stay that way.”

As the virus spreads through American communities, many leaders will face the same stubborn challenge: How, in a country that values its citizens’ medical privacy and autonomy, can authorities separate the sick from the well?

The question is an urgent one if public life is to resume.

Chinese cities solved this problem by giving infected people no choice. In the city of Wuhan, authorities realized that social distancing was not enough to rapidly bring the virus's reproduction rate down to near zero, which they felt was necessary to reopen schools and businesses.