An unseasonably early spike in respiratory syncytial virus cases among young children is pushing some hospitals to capacity.

RSV, as it's called, is a respiratory virus that mostly manifests as a mild illness with cold-like symptoms in adults but can cause pneumonia and bronchiolitis in very young children. It can be life-threatening in infants and older adults.

Most years, infections typically occur in the late fall and winter, often overlapping with flu season. But at least since last year, physicians have begun seeing surges starting during summer months.

Children's hospitals in the Washington, D.C. area, including Children's National Hospital, Inova Fairfax and Johns Hopkins in Baltimore, are at or near capacity, DCist reported.

Connecticut Children's Hospital in Hartford has had its pediatric in-patient beds full for the last few weeks, WTNH reported. With no indication of the spread slowing down, officials there are seeking the help of the National Guard and FEMA to set up tents in order to expand capacity.

In Texas, doctors at Cook Children's hospital in Fort Worth told ABC News they are treating some 300 RSV patients a day.

"Last year, more people were wearing face masks and children were more likely to stay home while sick," Dr. Laura Romano said in Cook Children's in-house publication.

"This year, parents are sending their children to daycare and school for the first time following two years of the pandemic. ... Children who haven't been previously exposed to respiratory viruses are getting sick," Romano said.

Health officials in King County, Wash., are also alarmed as they brace for more cases once winter hits. Dr. Russell Migita with Seattle Children's Hospital told King 5 News they are seeing about 20 to 30 positive cases every day, adding that those are "unprecedented" figures.

How RSV shows up

RSV symptoms are similar to a cold and can be harmless in adults, but the CDC says children under the age of 5 are the most affected group. According to the agency's data, each year approximately 58,000 children in that age range are hospitalized for RSV. The next most vulnerable group are adults over 65, in whom the infection causes 14,000 deaths a year.

RSV can lead to bronchiolitis, an infection that causes airways to become inflamed and clogged with mucus, making it difficult to breathe. If the infection travels to the lung sacs, it can result in pneumonia.

Dr. Sara Goza, physician and former president of the American Academy of Pediatrics, talked to NPR last year about how the infection presents in infants.

"A lot of the babies under a year of age will have trouble breathing. They stop eating because they can't breathe and eat at the same time. And they're wheezing, so they're in respiratory distress," Goza said.

Other symptoms include coughing, excessive sleeping and lethargy.

There is no vaccine to prevent RSV, but doctors are urging patients to get the flu shot. It doesn't prevent the infection but it could spare people from more aggressive symptoms and keep them from seeking medical attention at already strained hospitals.