An unseasonal surge of winter viruses is landing children in the hospital in the middle of summer.
The illnesses include severe colds, croup, which causes a severe cough, and respiratory syncytial virus, or RSV.
The rise worries already overwhelmed doctors as the school year approaches. Not only do they expect more spread of the winter viruses; they also anticipate a rise in COVID-19 cases among younger students. Virtually all elementary school-age children — that is, those younger than 12 — are unvaccinated, and some school districts have said they will not require masks in classrooms, regardless of vaccination status.
"We are all bracing for what's coming this respiratory season when kids go back to school," said Dr. Kristin Moffitt, an infectious diseases physician at Boston Children's Hospital.
The viruses circulating now spread mostly through respiratory droplets spewed when a person coughs or sneezes. Some, such as the virus that causes hand, foot and mouth disease, go from person to person via unwashed hands.
RSV can be particularly dangerous in young children and babies, as it can lead to pneumonia and bronchiolitis, an inflammation of the small airways in the lung.
"Normally, you just don't see RSV in the summertime at all," said Dr. David Kimberlin, co-director of the division of pediatric infectious diseases at the University of Alabama at Birmingham. The rise in such infections in June and July "exceeded our worst winters in terms of RSV hospitalization," he said.
Dr. Buddy Creech, a pediatric infectious disease expert at the Vanderbilt University Medical Center in Nashville, Tennessee, saw a similar rise in unseasonal viruses.
"Our children's hospitals are busy, predominantly because of the winter viruses that have come back with a vengeance," he said.
In June, the Centers for Disease Control and Prevention alerted doctors about the spread of RSV, especially in Southern states. Kimberlin said his pediatric intensive care unit almost ran out of beds because of the influx of RSV cases.
The team had to activate additional teams of doctors to keep afloat. "It was all hands on deck," he said.
While RSV cases are beginning to ease in the South, there is early indication that the virus is moving north.
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Dr. Sean O'Leary, an infectious diseases physician at Children's Hospital Colorado, said he is seeing a "big uptick" in RSV cases.
Other viruses, normally seen in the winter, are also landing children in the hospital this summer, including enteroviruses and parainfluenza 3, which causes croup.
"We're seeing cases of bronchiolitis, which is what RSV usually causes, but we're seeing it with the other viruses," O'Leary said.
And COVID-19 still remains a threat to children. According to the latest report from the American Academy of Pediatrics, about 130 new COVID-related hospitalizations in children have been reported since July 8.
Why typical winter viruses are popping up during the summer is not entirely clear. O'Leary, who is also vice chair of the Committee on Infectious Diseases for the American Academy of Pediatrics, suggested that one reason is that distancing and masking guidelines "are slowly but surely being loosened up."
"There is a lot more mixing among people than there had been," he said.
Kimberlin agreed. "These viruses thrive by social interactions," he said.
The repercussions of the atypical winter virus season of 2020-21 could last into the next season.
"Everything was thrown off by the pandemic," said Dr. Bernhard Wiedermann, an infectious diseases specialist at Children's National Hospital in Washington, D.C. "It's going to be hard to predict what's coming up for influenza and RSV and other things in the fall and winter."
The CDC director, Dr. Rochelle Walensky, warned about the probable increase in illnesses among schoolchildren Tuesday in testimony before the Senate Committee on Health, Education, Labor and Pensions.
"These kids have not been in school. They have not been with each other. I am worried about the upper respiratory infections," Walensky said, adding that coronavirus testing will be critical to determine whether symptoms are due to COVID-19 or another virus.
Doctors do expect an increase in COVID-19 cases in young children, for whom there is not yet a vaccine.
"I think we would be unwise to think that as the pandemic pushes forward that children aren't going to continue to be impacted because they are not vaccinated yet," Creech said.
"We have to focus on those most vulnerable around us," he said. "Right now, those are our children."
This story first appeared on NBCNews.com.