On the evening of Tuesday, March 10, 61 singers gathered for choir practice just north of Seattle. It was a standard rehearsal, as members sang in close quarters, shared snacks, and stacked chairs together at the end of the two-and-a-half-hour session. Five days later, the choir director sent an urgent email to the group. Several members had developed fevers, she said. The following Tuesday’s rehearsal was canceled. But by then, it was too late. Fifty-three of the 61 singers became ill, making for an “attack rate” of 87 percent. Three members of the choir were hospitalized. Two died.