Many attendees are older, which would put them at higher risk of severe complications from COVID-19. It’s not unusual for several individuals in the crowd to require medical attention when the temperature rises.

The rallies also typically draw supporters from surrounding towns and states. Some die-hard fans travel across the country from rally to rally like groupies for a band.

Dr. Ashish Jha, director of Harvard’s Global Health Institute, called the upcoming Trump rally “an extraordinarily dangerous move for the people participating and the people who may know them and love them and see them afterward.”

Trump supporters coming from neighboring cities and states could carry the virus back home, Jha said. “I’d feel the same way if Joe Biden were holding a rally.”