They were infected with coronavirus. They never showed signs.

At the moment, the Centers for Disease Control and Prevention allows for testing only symptomatic people who traveled to China recently or those who have had contact with someone who tested positive for coronavirus. (Officials have said the criteria may be re-evaluated.)

“We could be missing a great number of cases that don’t fit into those criteria,” said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

“I suspect there are a number of additional cases in this country that are transmitting this virus, just like we’re seeing in other countries. Absence of evidence is not evidence of absence.”

People who are infected but asymptomatic can spread disease efficiently. They are hardy and mobile. They have no reason to avoid crowds or kissing. They don’t know they are sick, and no one else does.