CHICAGO >> Younger people are making up a growing percentage of new coronavirus cases in cities and states where the virus is now surging, 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 ages 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 for the virus has dropped to 35, down from 65 in March.

And in Texas, where the governor paused the reopening process today as hospitals grow increasingly crowded, young people now account for the majority of new cases in several urban centers. In Cameron County, which includes Brownsville and the tourist town of South Padre Island, people under 40 make up more than half of newly reported cases.

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

The pattern is drawing notice from mayors, governors and public health officials, and comes as a worrisome sign for cities and institutions as they look to the fall. The rise in cases among younger people could complicate the plans of leaders who are eager to open schools and universities, resume athletic events and return to normal life and 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 across communities, including to more young people. But people in their 20s and 30s are also more likely to go out socializing, experts say, raising concerns that asymptomatic young people are helping to spread the virus to more vulnerable Americans at a time when cases are surging dangerously in the South and the West.

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

The United States recorded 36,975 new cases on Wednesday, a new high point in daily cases as the country confronted a new stage of the crisis two months after the previous high in late April. The resurgence is most immediately threatening states that reopened relatively early in the South and the West. Alabama, Florida, Oklahoma, South Carolina and Texas all reported their highest single-day totals this week, as did Montana and Utah, and cases were rising in 29 states today.

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 her apartment in San Marcos, Texas, and wearing a mask when going in and out of stores. At the one Black Lives Matter protest she attended, most people were in masks.

But after a particularly long week of juggling online summer classes and her job at an eye clinic, Carter took a risk one Saturday night in early June and met a friend at the Square, 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 and endured the hot Texas weather.

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

“We were told we could go out to bars,” she said, adding that she had been careful to quarantine since she learned that she had been exposed. “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.”

The new cases among young people may appear to be a departure from the early days of the pandemic when infections in nursing homes were spiraling out of control, and the virus appeared at higher rates among older people in New York City.

Experts cautioned that the seemingly new prevalence among young people may be, in part, a reflection of more widely available testing. But the growing numbers of people hospitalized in states like North Carolina and Texas also suggest increased transmission of the virus.

Even now, people younger than 50 are being hospitalized at a far lower rate than people older than that, according to CDC data.

While the effect of the coronavirus on younger people “may not be highly associated with hospitalization and death,” Redfield said, “they do act as a transmission connector for individuals that could in fact be at a higher risk.”

In Florida, which has emerged as a particularly concerning hot spot, reopened bars have been a source of contagion among young people. The state shut down the Knight’s Pub, a popular bar near the University of Central Florida in Orlando, after 28 patrons and 13 employees were infected.

In Miami-Dade County, the number of known coronavirus cases among 18- to 34-year-olds increased fivefold in a month, to more than 1,000, Mayor Carlos Gimenez said this week.

“They’re thinking they’re invincible,” he said, adding that many of the infected have no symptoms.

They are at higher risk, though, if they are overweight or have diabetes or other medical conditions, he said. About a third of the coronavirus patients at the public Jackson Health System were from that age group, and about half had a high body mass index, Gimenez said.

In Dallas County, people between the ages of 18-40 have made up 52% of newly reported cases since the beginning of June, a jump from the 38% that young people represented in March, according to county data.

At the same time, older people have begun to represent a smaller portion of the total number of people who test positive for the virus. In June, people over 65 have made up 8% of new confirmed cases in Dallas County, down from 16% in March.

The situation is particularly unsettling in Hays County, home to Texas State University in San Marcos. Coronavirus cases have surged since the beginning of June, to 2,100 this week, from 371 at the start of the month. People in their 20s now make up more than half of all known cases, officials said.

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

In Arcadia, Arizona, Ian Bartczak, who is 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 on a commercial strip near his home.

“It goes back to, what is a want and what is a need?” said 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, he said. He has turned down invitations to go out for sushi, and been puzzled by friends who chose to visit casinos.

“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?”