Although there are incidents of rampant spread, as happened on the cruise ship Diamond Princess, the coronavirus more often infects clusters of family members, friends and work colleagues, said Dr. David L. Heymann, who chairs an expert panel advising the World Health Organization on emergencies.

No one is certain why the virus travels in this way, but experts see an opening nonetheless. “You can contain clusters,” Dr. Heymann said. “You need to identify and stop discrete outbreaks, and then do rigorous contact tracing.”…

If it were possible to wave a magic wand and make all Americans freeze in place for 14 days while sitting six feet apart, epidemiologists say, the whole epidemic would sputter to a halt.

The virus would die out on every contaminated surface and, because almost everyone shows symptoms within two weeks, it would be evident who was infected. If we had enough tests for every American, even the completely asymptomatic cases could be found and isolated.

The crisis would be over.