Until several days ago, some bars and restaurants were still packed with St. Patrick’s Day crowds. Beaches were full. And it seemed as though many young adults were slow to take steps to curb the spread of the coronavirus.

“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.

Younger adults are generally more resilient than older people, but an increase in chronic health conditions among millennials, in their 20s and 30s now, has made them less hardy than they might think. They have seen rising levels of obesity and illnesses such as Type 2 diabetes and high blood pressure, as well as an increase in heart disease and some cancers. These are strongly associated with poor outcomes from coronavirus infection.

In addition, some 17 percent of men and 12 percent of women ages 18 to 44 smoke cigarettes, and 4 percent to 7 percent use e-cigarettes, according to national figures. Both increase the risk of respiratory illness.

How members of this generation will adapt to life during the pandemic might also dovetail with other problems. Compared with members of Generation X, in their 40s and 50s now, millennials have higher rates of behavioral health problems like depression — which affects one in 20 adults who are in their mid-30s, according to one report — and a significant number have alcohol and substance abuse problems, which are easily aggravated by social and economic turmoil and dislocation, experts say.

Millennials make up the largest share of the labor force and are a vital part of the economy, but they also face unique financial struggles. Many millennials juggle several jobs in the gig economy, and they are more likely than any other age group to be uninsured. Many are also burdened by student debt and contend with increasingly unaffordable housing. As a generation, they have borne the brunt of drug overdose deaths. Many have postponed marriage and starting a family for financial reasons.

While they have heard that by engaging in social distancing they can play an important role in reducing the spread of the coronavirus to others more vulnerable than they are, they are not immune.

Although the risk of hospitalization or dying from Covid-19, the disease caused by the virus, increases with each decade of life, according to the C.D.C.’s recent analysis of cases, the agency found that young adults can develop severe disease: Of 508 patients admitted to hospitals, 20 percent were 20 to 44 years old. Some younger people do die of the disease, at a rate of about one or two per 1,000 cases.

The report’s authors cautioned that their analysis was limited because much of their data was incomplete, and that information about chronic health conditions that affect the severity of disease was missing.

A report on millennials’ health by the Blue Cross Blue Shield Association, released last year, said chronic physical health conditions that are more common among millennials compared with Generation X members include high blood pressure, which affects more than one in eight adults in their mid-30s, and Type 2 diabetes, which affects one in 25. The report analyzed claims from the large insurer’s database.

Obesity, which has increased in prevalence for all Americans, is also considered a risk factor for the coronavirus, experts say.

But many news reports have consistently emphasized that the elderly and those in poor health are the most vulnerable and most likely to die.

“The message has been that if you’re younger and generally healthy, you’re going to be fine, and I think that’s the wrong message,” said Mila Clarke Buckley, 30, who writes about living with Type 2 diabetes on her blog, Hangry Woman, and creates sponsored content for FreeStyle Libre, a continuous glucose monitoring system. “It makes young people feel that invincibility they already feel about their health.”

Living with diabetes puts Ms. Buckley at heightened risk for severe illness if she were to become infected, she said, so she has stopped traveling out of town. She is also staying home more and practicing social distancing — even with her husband, who is still going out to work.

“He has a cold right now, so we are sleeping in separate bedrooms,” Ms. Buckley said from her home in Houston. “We know that if I get sick it’s a big deal — it may impact me a lot harder.”

Ms. Buckley said she had turned down a tempting invitation from her sister-in-law, who had invited the entire extended family over for dinner, now that restaurants were offering limited service. But the family is large, Ms. Buckley said, and she declined.

“We told her we don’t think it’s a good idea to gather, and perhaps she should rethink it,” she said.

To catch up with her friends, Ms. Buckley said, she made a date for a FaceTime Happy Hour for this week.

“We’ll each pick out our own favorite wine and jump on FaceTime at 7 o’clock,” she said.

But staying home and practicing social distancing may take a greater toll on young adults as they face additional mental health challenges, especially if they live alone or struggle with anxiety or depression, said Benjamin F. Miller, a psychologist who is the chief strategy officer for Well Being Trust, a national foundation focusing on mental and spiritual health.

Young adults are less likely than older adults to be married, and a 2018 Cigna survey of 20,000 Americans found that many reported feeling lonely and left out. Those who are 18 to 37 were more likely than older adults to report they had no meaningful relationships, did not share ideas and interests with anyone, felt isolated and did not feel close to anyone.

“Many of our millennials already feel socially disconnected, and this exacerbates those ongoing feelings these folks already had,” Dr. Miller said.

The Blue Cross report said that six of the top 10 conditions taking the heaviest toll on young adults were behavioral health conditions like substance abuse and mental health problems.

Like many millennials, Will Lanier, a 34-year-old from Austin, Tex., works at home, running the Out Foundation, a wellness and fitness organization for LGBTQ people that he founded. (A survivor of ulcerative colitis and colon cancer, he serves as a patient advocate on a Pfizer advisory board, for which he receives some financial compensation). He lives alone and worries about the desolation he might feel if classes at his CrossFit gym are shut down.

“FaceTime can only do so much, and human interaction is so important,” Mr. Lanier said.

While people often make a point of reaching out to older relatives or neighbors who live alone and may be lonely, he said, “people don’t check on young people.”

“It’s just me and my dog — I’ll go days without talking with someone,” he said. “If I slipped in the shower, it’d be days before anyone found me.”