On Saturday, President Trump met with Judge Amy Coney Barrett, the nominee to the Supreme Court, and others in the Oval Office. On Tuesday, he debated former Vice President Joseph R. Biden Jr. in an indoor hall, neither with a mask, talking at high volume and often without pause.
On Wednesday, Mr. Trump traveled to and from Minnesota on Air Force One along with dozens of others. On Thursday, the president appeared indoors before hundreds of supporters at a golf club in Bedminster, N.J.
On none of these occasions was the president wearing a mask. Often, neither were many in the room or on the airplane with him. All in all, conditions like these are a recipe for so-called superspreader events, in which a single infected person transmits the virus to dozens of others, research has shown.
Experts may never know how the president was infected or whom he may have infected, but the timing raises the possibility that he could have been infected last weekend, scientists said. Most people develop symptoms about five days after being infected with the virus, so exposure over the weekend would fit with reports that the president was showing early symptoms on Wednesday and Thursday.