Infected young people seed waves of the virus that sicken older people, C.D.C. report says.

As millennials mingled in bars and restaurants over the summer, and students returned to college campuses, coronavirus infections surged among young adults.

From June through August, the incidence of Covid-19 was highest among adults aged 20 to 29 years old, according to research published on Wednesday by the Centers for Disease Control and Prevention. Young adults accounted for more than 20 percent of all confirmed cases.

But the infections didn’t stop with them, the researchers found: Young adults also seeded waves of new infections among the middle-aged, and then in older Americans.

The new data show that outbreaks linked to parties, bars, dormitories and other crowded venues are hazardous not just to the twenty-somethings who are present, but to more vulnerable Americans with whom they are likely to come into contact.

College campuses have become a particular threat. According to a database maintained by The New York Times, there were more than 88,000 coronavirus infections reported on nearly 1,200 campuses as of early September.

At a Congressional hearing on Wednesday, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, warned against sending home students from colleges experiencing outbreaks.