Public health officials raced to figure out who else might be walking around infected. The club provided a list of 100 bridge players, and the county set up a hotline for people who suspected they had been exposed.

The circle kept growing. One bridge player had gone to a choir practice: Add another 150 names to the list. The county’s four-person epidemiology staff got so swamped that it drafted coroner’s investigators to start calling.

Pete Coggeshall, 80, was one name on the list. He sat beside the 83-year-old woman on March 3. Days later, he started to feel a strange cold coming on. Coca-Cola, which he loved, tasted strange. He avoided the common communion cup during Sunday services at the Rock of Ages Lutheran Church, and then felt worse after he got home. He ended up in a hospital for three days. “It was like somebody hit me with a brick,” he said.

Mr. Coggeshall and other members have puzzled over how the infection circulated through the games — The plastic boards holding the cards? An invisible mist in the air? The snack table stocked with supermarket cold cuts? — but they said there was no way of knowing for sure.

The club started its own list. A group that was meticulous about tallying scores and the “master points” that players earned over a lifetime now had other wins and losses to chart. They tracked who was waiting on a coronavirus test. Who was on a ventilator. Who was fading.