“The combination of the two factors makes Biden a close contact, even if the letter of the law doesn’t consider him one,” Dr. Marr said.

Mr. Biden tested negative for the virus twice on Friday, the Biden campaign said. He was not tested on Saturday and left his home for part of the day, going to a theater to participate in a virtual event and a church to attend Mass. The campaign announced his latest negative test result on Sunday night. Pressed on CNN about the frequency of Mr. Biden’s coronavirus tests, Ms. Sanders said the candidate was “tested before we travel.”

In the coming days, the Biden campaign is aiming to balance health precautions with continued travel in Florida on Monday and in Arizona on Thursday. Yet Mr. Trump’s positive test has plainly injected an extraordinary measure of uncertainty into the race.

“We only have a few weeks left to the election, and they want Joe Biden to have the ability to present himself to the American public,” Dr. Redlener said. “And the president’s recklessness has thrown things off track, and the Biden campaign is frustrated by that.”

But whatever his travel schedule, Mr. Biden should be tested every day at least until Oct. 13, two weeks from the night of the debate, and probably even after that given his interactions with people on the campaign trail, experts said.

“Getting a negative test does not necessarily mean that you’re not infected,” said Juliet Morrison, a virologist at the University of California, Riverside. Early in the infection, the amount of the virus may be too low to trigger a positive result. “You have to keep being diligent, and assuming that you may have actually been infected, so you need to be testing on a daily basis,” she said.

A rapid test like the one used by the White House can miss infections, especially early on in the course of the illness. Mr. Biden should instead be given the more rigorous P.C.R. test, which can deliver results in a few hours when not delayed by bureaucratic logjams, said Virginia Pitzer, an epidemiologist at the Yale School of Public Health.

Mr. Biden’s campaign should make the results of those tests public as soon as they are available, she said: “Transparency is key.”

Thomas Kaplan reported from Washington, and Apoorva Mandavilli and Katie Glueck from New York. Sydney Ember contributed reporting from Connecticut.

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