"I stand by my words," says Dr. James Phillips, the Walter Reed physician who said the president's decision to drive by supporters while being treated for COVID-19 endangered his security detail.

"Agents are already worrying about guns and knives and bombs," a former Secret Service agent says. "Now they have to worry about COVID-19."

There's a long history of military doctors treating U.S. presidents. This does create the unusual situation where the president, as commander-in-chief, is the doctor's ultimate boss.

The Army is working with private industry to create a coronavirus vaccine, but also working on its own. The military service has a history of creating vaccines and making medical breakthroughs.