In Oxford, Mississippi, summer fraternity parties sparked outbreaks. In Oklahoma City, church activities, fitness classes, weddings and funerals seeded infections among people in their 20s, 30s and 40s. In Iowa college towns, surges followed the reopening of bars. A cluster of hangouts near Louisiana State University led to at least 100 customers and employees testing positive. In East Lansing, Michigan, an outbreak tied to a brew pub spread to 34 people ages 18 to 23.

“The virus hasn’t changed. We have changed our behaviors,” said Ali Mokdad, professor of health metrics sciences at the University of Washington in Seattle. “Younger people are more likely to be out and taking a risk.”

In Florida, young people ages 15 to 34 now make up 31% of all cases, up from 25% in early June. Last week, more than 8,000 new cases were reported in that age group, compared with about 2,000 among people 55 to 64 years old. And experts say the phenomenon cannot be explained away as simply the result of more testing.