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President Trump called Democratic presidential nominee Joe Biden a "dumb guy" and pointed to his history in politics at a rally in Harrisburg, Pa., on Sept. 26. (Video: The Washington Post)

When he first learned he had tested positive for the corona­virus, President Donald Trump was already aboard Air Force One, en route to a massive rally in Middletown, Pa.

With him on the plane that Saturday evening were dozens of people — senior aides, Air Force One personnel, junior staffers, journalists and other members of the large entourage typical for a presidential trip — all squeezed together in the recirculating air of a jetliner.