American adults of all ages — not just those in their 70s, 80s and 90s — are being seriously sickened by the coronavirus, according to a report on nearly 2,500 of the first recorded cases in the United States.

The report, issued Wednesday by the Centers for Disease Control and Prevention, found that — as in other countries — the oldest patients had the greatest likelihood of dying and of being hospitalized. But of the 508 patients known to have been hospitalized, 38 percent were notably younger — between 20 and 54. And nearly half of the 121 patients who were admitted to intensive care units were adults under 65, the C.D.C. reported.

“I think everyone should be paying attention to this,” said Stephen S. Morse, a professor of epidemiology at Columbia University’s Mailman School of Public Health. “It’s not just going to be the elderly. There will be people age 20 and up. They do have to be careful, even if they think that they’re young and healthy.”