Younger Americans eager to get back to their social lives are increasingly responsible for the spread of the coronavirus, risking their own health and that of their family and friends under what health experts say is the misguided impression that the virus cannot cause them harm.

Health departments across the country are reporting that younger people are making up larger shares of the total number of those infected with the virus. The greater infection rates among young people are occurring both in states that are getting a handle on their outbreaks and those that are not.

“In these trends, we are seeing the impact of our collective decisions. We are jeopardizing the gains we made as a state,” Washington state Health Secretary John Wiesman said Friday, pointing to an increase in hospitalizations among people between the ages of 20 and 39. “[T]he actions each one of us takes now will determine what happens next.”

In early June, just 10 percent of those who tested positive in Rhode Island were in their 20s. By the end of the month, that share had doubled. The average age of a Rhode Islander who tests positive fell from 47.5 years old to 39.2 years old in a week.

Maryland Gov. Larry Hogan (R) said this week the percentage of those under 35 testing positive for the virus is now 84 percent higher than it is for those over 35.

In New Mexico, 44 percent of those who are testing positive for the virus are under 30 years old, according to state health data. In Illinois, there are more infections among people aged 20-29 than among any other age group. In California, those in their 20s make up the largest cohort of cases, followed closely by people in their 30s.

“There is a sense that a lot of young people, you’re young, think you’re invincible,” California Gov. Gavin Newsom (D) said at a June 24 press briefing. “That can be a selfish mindset.”

There are signs, too, that even children are vulnerable to the disease. More than 10 percent of confirmed cases in Arizona, Washington and Tennessee are among those under the age of 20, an analysis by Bloomberg found.

Public health experts think younger people may be under the impression that the coronavirus can do them no harm, after early signs showed that the virus was less likely to lead to severe outcomes among young adults. After months of disruptive lockdowns, they may be more interested in returning to some semblance of normalcy than in continuing to make the sacrifices necessary to bring the virus under control.

But while younger people are at less risk than those who are older, they are not immune. Data from the Centers for Disease Control and Prevention show that about 3,000 people in the United States under the age of 45 have died of COVID-19. 

“One of the pieces of good news within this pandemic is that thankfully younger people are at lower risk of having severe disease, of being hospitalized and of dying from [COVID-19]. But the risk isn’t zero,” said Richard Besser, a former acting director of the Centers for Disease Control and Prevention who now heads the Robert Wood Johnson Foundation.

The risk extends beyond death, too. Preliminary studies have also hinted at the potential for long-term damage to the hearts, lungs and brains of those who survive the disease.

Public health officials are increasingly addressing their concerns to young people specifically.

“You have to have responsibility for yourself but also a societal responsibility that you're getting infected is not just you in a vacuum. You're propagating the pandemic,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Thursday, addressing young people in an interview with Facebook CEO Mark Zuckerberg.

What worries public health experts more than a surge of death among young people is that young people who are infected, who may not even know it, risk becoming vectors of transmission themselves. While they might not experience the worst symptoms of COVID-19, they might expose parents, grandparents, even friends or relatives who might have the underlying conditions that contribute to more severe disease.

“Young people don’t just stay with young people. Young people come home to older relatives who may be at higher risk, because of medical problems or simply due to their age,” Besser said. 

In response to the growing number of younger people infected, some areas have closed bars and restaurants once again, in hopes of driving younger people out of densely crowded facilities where transmission may take place.

In his letter, Maryland’s Hogan warned local officials to crack down on establishments violating an executive order he issued requiring social distancing at eating establishments. 

“At least 12 states have already moved to re-close bars and restaurants — we do not want to be forced to take the same action here in Maryland,” Hogan wrote as he urged counties to crack down on scofflaw businesses. “Our continued economic health and recovery depend on the active and aggressive local enforcement of these critical public health measures.”

Even with bars closed in many parts of the country, health officials are tracing clusters of cases back to house parties, where dozens of people can be infected by just one person who is sick. In recent weeks, health officials in Michigan, Minnesota, Pennsylvania and Oregon have identified house parties as the sources of large-scale transmission.

The house parties show that government restrictions meant to stamp out the virus only work if residents comply. Those who say abiding by the onerous rules — whether they are limits on crowd size or the use of masks in public — recall another group whose actions put public health at risk, smokers.

“Some of that reminds me of the discussion around second-hand smoke,” Besser said. “When smokers used to say, ‘Hey it’s my right to smoke,’ and we said, ‘Well you’re actually putting other people at risk.' ”