The rising number of COVID-19 cases in states across the country is due in large part to more young people contracting the virus, raising alarms among public health officials.

The spikes suggest young adults are both more likely to hold front-line service jobs that put them at risk and more likely to ignore some of the social distancing practices advised by health experts.

The most troubling hot spots are now concentrated in Sun Belt states such as Arizona, California, Florida, North Carolina and Texas. All five of those states have reported more than 1,000 new cases per day this week, making them the only five states to break the four-digit barrier during that period.

Texas Gov. Greg Abbott (R) said the majority of new cases in his state are coming from residents who are under 30 years old. He said many of the cases are tied to "bar-type settings" where people have been congregating since Memorial Day weekend.

"What we're seeing there is that people of that age group, they're not following these appropriate best health and safety practices," Abbott said in an interview this week with KLBK, a McAllen television station. "They're not wearing face masks. They're not sanitizing their hands. They're not maintaining the safe distancing practices. And as a result, they are contracting COVID-19 at a record pace in the state of Texas."

In California, 44 percent of recent coronavirus cases occurred in those under the age of 35, according to a study by George Lemp, director of the University of California's HIV/AIDS Research Program.

California officials said they have seen an increasing number of younger people contracting the virus but that it was likely a function of a rising number of diagnostic tests administered to those who show few if any symptoms.

"In the first months of the outbreak, when testing was more limited, we were largely testing individuals who were most vulnerable, including those who were hospitalized or had arrived at an emergency room with severe symptoms as well as high-risk individuals with symptoms in congregate facilities where an outbreak might have been occurring," said Ali Bay, a spokeswoman for the state health department. 

"We have significantly expanded the availability of testing to cover a broader set of communities and environments and are now seeing a lot of younger people get tested, including people who are in higher risk work environments, others who are returning to non-essential jobs, and people who want to know their status," she added. 

Bay also pointed to statistics showing that the number of hospitalizations remains stable and that the percentage of tests coming back positive has dropped.

Clusters of new cases among young people have emerged around the University of Central Florida in Orlando and in Mobile County, Ala. Other cases have been tied to a party in Uinta County, Wyo., and fraternity parties around the University of Mississippi in Oxford. 

Figures compiled by the Centers for Disease Control and Prevention show that about 40 percent of the coronavirus cases confirmed since early March have been among people between the ages of 18 and 49, the narrowest range their data covers. The percentage of younger people as a share of the total population who have tested positive has steadily risen, from 32 percent in the first week of March to more than half over the past week. 

"We know that younger people are less likely to show symptoms — so earlier on when testing was only happening if you had symptoms, the skew looked to be toward older people primarily getting COVID-19, which was likely not the case. We just were not testing enough," said Abraar Karan, an internist at Brigham and Women's Hospital and Harvard Medical School. "Countries that were testing everyone already showed a skew toward younger people driving the epidemic months ago."

Karan pointed to studies from Japan that showed people between the ages of 20 and 39 made up a majority of coronavirus cases. He said younger people are more likely to venture out after months in lockdown, in part because they do not think they are at risk as much as older people are.

"Younger people are more likely to be back out and about, probably because they perceive their personal risk to be less than older people do. They are likely to have more person-contacts in a day as well by virtue of being more socially and economically active. They are also more likely to be attending certain types of high risk venues, such as bars and clubs," he said in an email.

Others said younger workers are also more likely to venture out for economic reasons. They are more likely to have jobs in the service industry, putting them at risk of contracting the virus through contacts with restaurant, convenience store and grocery store patrons. 

"Rent, car payments, grocery bills didn't really slow down during the quarantine, and hourly employees' wallets are empty," said Nita Bharti, a biologist at the Center for Infectious Disease Dynamics at Penn State University. "The government's inadequate economic relief response made quarantine wholly unsustainable for the people who most needed support during the shutdowns." 

The increasing number of younger people contracting the virus has not led to a spike in younger people in the hospital or dying. Research shows the virus is still much more likely to have the worst outcomes for older people and those who have underlying conditions.

But experts are concerned that those who contract the virus can put others at risk, either fellow young people or parents, grandparents and others who might face more severe consequences.

While more testing does lead to more positive tests, an increase in the number of people who are tested does not completely account for the total rise in confirmed cases, according to Marta Wosinska, deputy director of policy at the Duke-Margolis Center for Health Policy. 

That suggests the virus continues to spread at faster rates in some places. 

"You do expect that if you do more testing you might find more cases, but that's actually not the case in most states here," Wosinska said. "In states like Texas, Florida, Georgia, North Carolina and Arizona, you do see an increase in testing, but you see the cases rising at a much faster rate.”