In early April the Trevecca Center for Rehabilitation & Healing in Nashville received an urgent call. At the time, Tennessee had only 3,000 coronavirus cases, compared with more than 100,000 in the state of New York. But the caller, the nursing director at a nearby kidney dialysis center, was worried. She said one of the home’s residents had been given a routine test before an appointment and had tested positive for the coronavirus. She said she believed that Trevecca, a 240-bed nursing home, might have an outbreak on its hands. Trevecca’s managers were busy, she was told, so she left a message.