Americans are afraid of the health risks from Covid-19. But many also rightfully worry about when the intense restrictions on movement and activity will end. Addressing both concerns will require setting up a robust testing system that can catch outbreaks before they become difficult to manage.

April will be a hard month as the epidemic spreads across the nation. The priorities are maintaining the health-care system, preserving life, protecting the vulnerable, and supporting the economy. The epidemic may calm down by July...