Dr. Jack Rowe, a professor of health policy and aging at Columbia University, said he didn’t understand why Congress would ignore the threat.

“There are 535 of these people between the two chambers — you can’t tell me there aren’t any of them that don’t have chronic disease who would be at special risk,” he said. “These people should be excused. They should be sitting at home looking at their computers, listening to the debate and voting, and that would send the right message.”

A handful of Republican legislators who had direct contact with an infected man at the Conservative Political Action Committee conference last week have voluntarily quarantined themselves to prevent the spread of coronavirus. Senator Ted Cruz of Texas, Representative Matt Gaetz of Florida, Representative Doug Collins of Georgia, Representative Paul Gosar of Arizona and Representative Mark Meadows of North Carolina are staying away from the Capitol. So are the Democratic representatives Julia Brownley of California and Don Beyer of Virginia, who had contact with infected persons elsewhere.

On Wednesday, Congress’s attending physician issued a memo to legislators with recommendations on how to lower their risk of catching or spreading the virus. It said they should avoid large group meetings and screen visitors for signs of respiratory illness. It also said: “Members should adopt a stance that avoids close direct contact with other individuals, such as shaking hands, giving/receiving hugs, taking selfies.” The memo also recommends frequent hand washing.

Although some lawmakers have expressed private alarm that their oldest colleagues are traveling, meeting constituents and congregating on the House floor, they have publicly shown deference. Senator Chris Murphy, Democrat of Connecticut and one of the younger members at 46, said every member of Congress knows the job has risks.