“I kept hearing, ‘Eighty percent of cases are mild,’” said Christian Heuer, 32, of Los Angeles, who tested positive for the virus last week and has been running a low-grade fever for six days. “But this is not just a sniffly runny nose. It’s the real deal. You’re really sick.”

His girlfriend, Natasha Wynnyk, 28, felt fine for several days after Mr. Heuer got sick, and she thought she might be impervious to infection. Then her fever spiked on Monday evening, and she started experiencing severe and sharp aches in her back, joints and fingers, which she compared to a feeling of being stabbed.

The couple are part of a worrying trend suggesting that young people may have contributed to the pandemic’s spread in the United States and other countries by going about business as usual for too long, perhaps believing that being young and healthy protected them from infection. But preliminary figures released on Wednesday by the Centers for Disease Control and Prevention show that adults ages 20 to 44 represent nearly one-third of U.S. coronavirus patients whose ages are known.