Some US hospitals are seeing an increase in RSV and higher levels of “walking pneumonia” among young children, despite overall respiratory disease activity remaining low nationally.

Cook Children’s Medical Centers in Texas reported a “steep increase” in the number of children visiting the emergency room for respiratory illnesses.

On Tuesday, the health system’s Fort Worth headquarters alone had 572 patients — a near-record number — in the emergency department. Officials said the increase in hospital visits is due to the spread of RSV and walking pneumonia in the Dallas-Fort Worth area.

In addition, University of North Carolina Hospitals reported 40 cases of hospital-acquired pneumonia in the last week of October, compared to no cases during the same period last year.

Infectious disease specialists say that while parents should remain vigilant when it comes to the spread of respiratory illnesses, this season could also mark a return to typical seasons before the COVID-19 pandemic.

“It’s the calm before the storm,” told ABC News Dr. Peter Chin-Hong, professor of medicine and infectious disease specialist at the University of California, San Francisco. “It feels like there’s so much going on … that we almost forget about respiratory viruses, but they’re very regular. They kind of fall back into the normal pre-pandemic cadence.”

Walking pneumonia with a cyclical increase

So-called “walking pneumonia” is a bacterial infection of the respiratory tract caused by the bacterium Mycoplasma pneumoniae (M. pneumoniae), according to the Centers for Disease Control and Prevention.

The infections are generally mild, and people may seem better than expected for those with a lung infection, hence the term walking pneumonia, the CDC said. However, sometimes serious complications occur that require hospitalization.

For the week ending Nov. 2, the most recent for which CDC data is available, 2.8 percent of all pneumonia-related emergency room visits led to a diagnosis of M. pneumoniae, up from just 2 percent at the end of the month September.

Rates were highest among those 1 year of age and younger, accounting for 7.8 percent of all pneumonia-related emergency room visits with a diagnosis of M. pneumoniae for the week ending Nov. 2.

Experts say mycoplasma infections are cyclical in nature and tend to increase every three to seven years.

“Like other respiratory viruses, there wasn’t much exposure during the pandemic years because of social distancing and people staying away from each other, so we’re catching up,” Chin-Hong said. “Mycoplasma can also be one of these things where you see the perfect storm. So it comes back every four or five years and people go back to their normal lives.”

RSV activity is returning to pre-pandemic levels

Respiratory syncytial virus, or RSV, is a common respiratory virus, with most children affected by their second birthday, according to the CDC.

Although it usually causes mild, cold-like symptoms and most people recover within a week or two, it can be a serious infection for infants and older adults, leading to hospitalization, the CDC said.

Current levels remain lower than those seen at the same time in recent seasons.

“While RSV activity is increasing in some regions, particularly among young children, it is important to remember that we are witnessing a return to more typical pre-pandemic patterns,” said Dr. John Brownstein, epidemiologist and chief innovation officer at Boston Children’s. Hospital and an ABC News contributor.

“This year’s season starts later and progresses more slowly compared to past years, which have been marked by earlier and more severe outbreaks,” he added.

Brownstein pointed out that while national levels remain low, certain regions of the US South and US East are seeing localized increases, particularly among young children.

When to go to the emergency room

Chin-Hong said if symptoms are mild, including low-grade fever, dry cough, sore throat, headache and mild aches, parents can keep their children home and contact their primary care provider with any questions.

However, when symptoms become more severe, including shortness of breath, a prolonged fever, or lethargy, then it may be time to visit the emergency room.

If you have “a child who’s been coughing for more than seven days and maybe won’t shake it, then you’re worried about something like walking pneumonia,” Chin-Hong said.

How to prevent RSV and walking pneumonia

To prevent RSV, there are three vaccines approved for adults age 60 and older, as well as for some adults ages 50 to 59 who are at higher risk. There is also a vaccine available for pregnant women between 32 weeks and 36 weeks of pregnancy.

For babies under eight months, two monoclonal antibody products are available. Monoclonal antibodies are proteins made in a laboratory that mimic the antibodies the body naturally creates when fighting an infection.

“For walking pneumonia or mycoplasma, there’s no vaccine for that, but by getting vaccines for RSV, COVID (and) flu, you reduce the likelihood of co-infections that can make things worse,” Chin-Hong said.

Experts also recommended applying lessons learned during the COVID-19 pandemic, such as wearing a mask in poorly ventilated spaces.

“Practicing good hygiene, such as frequent hand washing and covering coughs and sneezes, remains critical in preventing the spread of respiratory infections,” Brownstein said.