MOUNT VERNON, Wash. — With the coronavirus quickly spreading in Washington state in early March, leaders of the Skagit Valley Chorale debated whether to go ahead with weekly rehearsal. The virus was already killing people in the Seattle area, about an hour’s drive to the south. But Skagit County hadn’t reported any cases, schools and business remained open, and prohibitions on large gatherings had yet to be announced. On March 6, Adam Burdick, the choir’s conductor, informed the 121 members in an email that amid the “stress and strain of concerns about the virus,” practice would proceed as scheduled at...