The numbers don’t tell the whole story. Nine Americans have died of Covid-19, and the Centers for Disease Control and Prevention confirms more than 100 cases in 15 states. But there are probably already several thousand Americans who have been infected. For now, the average person’s risk is low in a nation of 330 million, but that may soon change. The highest risk is for those who live in regional hot zones such as Washington state or Northern California.

Measures that limit social activity may soon be in place. China tried...