Dr. Daniel Winetsky, an infectious diseases fellow at Columbia University in New York, said his advice to his own parents, who live across the country in San Francisco, has shifted dramatically. A week ago, he said, he was reassuring them about their safety, even encouraging them to go ahead with a trip they were planning to the Florida Everglades with a small tour group.

Over the weekend, his fears about the pandemic rose, and by Tuesday not only was he telling them not to go, but he also was advising them to reduce to a minimum the number of people they came into contact with. Visits with grandchildren are verboten.

Dr. Winetsky told his mother, Carol, who is 73 and has asthma, to stop meeting with her biweekly knitting group. And he instructed his father, Hank, who has had two coronary stents, not to attend either of his two book group meetings.

His mother continues to go to the grocery store, while avoiding crowded places like Costco.