(NEW YORK) -- Some hospitals in the U.S. are seeing an increase in RSV and higher levels of "walking pneumonia" among young children despite overall respiratory illness activity remaining low nationally.
Cook Children's Medical Centers in Texas reported a "steep increase" in children visiting the emergency room due to respiratory-related illnesses.
On Tuesday, at the health system's Fort Worth location alone, there were 572 patients -- a near-record-high number -- in the emergency department. Officials said the increase in hospital visits is due to a spread of RSV and walking pneumonia in the Dallas-Fort Worth area.
Additionally, University of North Carolina hospitals reported 40 walking pneumonia cases in the last week of October compared to no cases the same time last year.
Infectious disease specialists say that although parents should stay alert when it comes to respiratory illnesses spreading, this season also may also be marking a return to typical seasons seen before the COVID-19 pandemic.
"It's the calm before the storm," Dr. Peter Chin-Hong, a professor of medicine and infectious disease specialist at the University of California, San Francisco, told ABC News. "It feels that there's so much going on … that we almost forget about respiratory viruses, but they're very regular. They're kind of falling back into normal pre-pandemic cadence."
Walking pneumonia seeing cyclical increase
So-called "walking pneumonia" is a respiratory tract bacterial infection caused by the bacteria Mycoplasma pneumoniae (M. pneumoniae), according to the Centers for Disease Control and Prevention.
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. Sometimes serious complications, however, emerge requiring hospitalization.
For the week ending Nov. 2, the latest for which CDC data is available, 2.8% of all pneumonia-associated ED visits led to an M. pneumoniae diagnosis, up from just 2% at the end of September.
Rates were highest among those ages 1 and younger, making up 7.8% of all pneumonia-associated ED visits with an M. pneumoniae diagnosis for the week ending Nov. 2.
Experts say mycoplasma infections have a cyclical nature and tend to surge every three to seven years.
"Just like other respiratory viruses, there was not a lot of exposure during the pandemic years because of social distancing and people staying away from each other, and so we're catching up to that," Chin-Hong said. "Mycoplasma may also be one of these things where you're kind of seeing the perfect storm. So, it comes back every four or five years and people are getting back to their normal lives."
RSV activity 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 typically causes mild, cold-like symptoms and most people recover in one to two weeks, it can be a serious infection for infants and older adults, resulting in hospitalization, the CDC said.
Current levels remain lower than those seen at the same time during the past few seasons.
"While RSV activity is increasing in certain regions, particularly among young children, it's important to remember that we're seeing a return to more typical pre-pandemic patterns," said Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor.
"This year's season is starting later and progressing more slowly compared to the past few years, which were marked by earlier and more severe outbreaks," he added.
Brownstein pointed out that while national levels remain low, certain regions in the southern U.S. and eastern U.S. are seeing localized surges, especially 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, headaches and mild aches and pains, parents can keep their children home and contact their primary care provider with any questions.
However, when symptoms become more severe including difficulty breathing, a long-lasting fever or lethargy, then it might be time to visit the emergency room.
If you have "a child who's having a cough for more than seven days and maybe they're not shaking that off, then you worry 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 ages 60 and older as well as some adults between the ages 50 and 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, there are two monoclonal antibody products available. Monoclonal antibodies are proteins manufactured in a lab 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] influenza, you reduce the probability 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 spaces with poor ventilation.
"Practicing good hygiene, like frequent hand-washing and covering coughs and sneezes, remains essential in preventing the spread of respiratory infections," Brownstein said.