“These conditions can limit underlying reserve and lead to worse outcomes when older people become severely ill, which taxes all organ systems,” said Dr. Annie Luetkemeyer, an infectious diseases specialist at Zuckerberg San Francisco General Hospital.

“For example, diabetes can make it harder to fight infection, and underlying heart or lung disease may make it more difficult for those organs to keep up with demands created by a serious Covid-19 infection,” she said, referring to the syndrome caused by the new coronavirus.

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. With her son’s permission, she still goes to physical therapy for a back injury, but she is careful to make sure the therapist washes her hands and that the equipment gets wiped down with disinfectant.

What about nonessential doctor’s appointments?

Some experts are recommending that older adults at risk cancel nonessential doctor’s appointments, including wellness visits. Telemedicine sessions, if available, are often a reasonable substitute.