CHICAGO • Younger people are making up a growing proportion of coronavirus cases in cities and states across the US, a trend that has alarmed public health officials and prompted renewed pleas for masks and social distancing.

In Arizona, where drive-up sites are overwhelmed by people seeking coronavirus tests, people aged 20-44 account for nearly half of all cases. In Florida, which breaks records for new cases nearly every day, the median age of residents testing positive has dropped to 35, down from 65 in March. And in Texas, young people now account for the majority of new cases in several urban centres.

"What is clear is that the proportion of people who are younger appears to have dramatically changed," said epidemiology professor Joseph McCormick, from the UTHealth School of Public Health in Brownsville. "It's really quite disturbing."

The pattern comes as a worrisome sign for cities and their leaders who are eager to return to a fully functioning economy.

The increases could reflect a simple reality: Since many states have reopened bars, restaurants and offices, the coronavirus has been allowed to spread more widely.

But people in their 20s and 30s are also more likely to go out socialising, experts say, raising concerns that asymptomatic young people are helping to spread the virus to more vulnerable Americans.

Dr Robert Redfield, the director of the Centres for Disease Control and Prevention, said on Thursday that younger people have helped fuel the increase in known coronavirus infections - and that in the past, many of those infections went undiagnosed.

"Our best estimate right now is that for every case that was reported, there actually were 10 other infections," he said.

The United States recorded a new daily high of 36,975 fresh cases on Wednesday, as the country confronted a new stage of the crisis after its previous high in late April.

Ms Adriana Carter, 21, is among the newly infected.

For many weeks this spring, she said, she took steps to limit her exposure, eating many of her meals at home in San Marcos, Texas, and wearing a mask outside. At the one Black Lives Matter protest she attended, most people were in masks.

But Ms Carter took a risk one Saturday night this month and met a friend at a popular bar district downtown. Though they were careful to avoid the most crowded spots, they chose not to wear masks as they sipped drinks inside.

Days later, her friend woke up ill. Both tested positive for the virus.

"We were told we could go out to bars," she said. "It's very unusual for anyone in their 20s to stay at home all the time - not giving any excuses or anything, but I just think we are all just trying to do the best we can."

Experts cautioned that the seemingly new prevalence among young people may be, in part, a reflection of more widely available testing.

Some believe that a decision by older people to stay home to avoid the virus may also help explain why young people appear to be an increasing portion of new cases.

The rising infections have set many people on edge, including some in their 20s and 30s.

In Arcadia, Arizona, Mr Ian Bartczak, 31, said he did not feel comfortable dining out at restaurants and was dismayed to see crowds of young people squeezing onto patios and bars near his home.

"It goes back to, what is a want and what is a need?" said Mr Bartczak, who works for an education technology company.

"Did you have to go to a big swimming party or El Hefe nightclub with your friends?"

His point of view has created awkwardness with some friends after he turned down invitations to go out, he said.

"It's affected some of my relationships because I won't see them or get kind of angry," he said. "How are you not willing to help the old lady behind you who could have a poor immune system? Or help lower our cases so we can increase our economy?"

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