LONDON — In Munich, normally brimming with boisterous crowds for Oktoberfest this month, the authorities just banned gatherings of more than five people. In Marseille, France, all bars and restaurants will be closed next Monday. And in London, where the government spent weeks urging workers to return to the city’s empty skyscrapers, it is now asking them to work from home.

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.