The power of the coronavirus is not that it’s particularly lethal or especially easy to catch from someone else, but rather that it’s stealthy. Infected people can pass it on to others before they realize they’re sick, or even if they never develop symptoms at all, making them unwitting spreaders. The defense against the virus’s insidiousness is thorough testing, to reveal where it is and might be in order to stop its spread, and to determine where it has been, which may help restart economies.