Infections with the illness known as “walking pneumonia” or “white lung pneumonia”  have been spreading at unusually high levels in young kids, emergency room data suggests, a year after a surge of such cases filled hospitals overseas.

The worst rates of the illness, caused by the bacteria Mycoplasma pneumoniae, are in young children ages 2 to 4 years old, according to Centers for Disease Control and Prevention figures shared with CBS News. 

Close to 7% of emergency room visits with pneumonia in this age group were diagnosed with the bacteria through late September.

This has “dropped slightly” from a peak of more than 10% in August, a CDC spokesperson said.

“The increase in 2–4-year-olds is notable because these infections have historically been thought to affect school-age more than younger children,” the agency said.

The figures come from the CDC’s National Syndromic Surveillance Program, which crunches numbers from emergency rooms. It echoes an increase reported by testing company BioFire Diagnostics, tallying trends that are now more than 14 times higher than this time last year.

A CDC spokesperson said that levels are the worst right now across two regions in the middle of the country, from Texas through Iowa.

Multiple other states have also now warned doctors about surges from Mycoplasma pneumoniae.

Wisconsin’s health department said Friday that it had received reports of increasing “unusual pneumonia cases” in kids and young adults from doctors around the state. 

Health officials in Illinois announced Thursday that they had tracked “several clusters reported in schools throughout the state,” alongside increases in data from testing labs.

Several hospitals have also reported a spike in kids with pneumonia, blamed on Mycoplasma pneumoniae alongside other germs like rhinovirus and enterovirus. Those bugs are often causes of the common cold but can also cause more serious diseases.

Trends of rhinovirus and enterovirus reported to the CDC have accelerated in recent weeks, nearing peaks seen during previous fall-time waves. 

“It’s likely to worsen with pollen and mold counts rising, colder weather keeping everyone inside and the holidays bringing people together,” Virginia-based health system VCU Health said last week.

Not all hospitals have seen an unusual surge. In Pennsylvania, where the state’s health department recently warned that cases of Mycoplasma pneumoniae were high, multiple health systems told CBS News they had not seen a spike yet.

Dr. Marian Michaels, professor of pediatrics and surgery at Children’s Hospital of Pittsburgh, said it was “too early to tell for sure” if this year’s wave would amount to an unusual increase in hospitalized patients.

Michaels was the co-author of a report published earlier this year by the CDC which found kids sickened with the bacteria increased last year but remained lower than before COVID-19.

“The numbers are perhaps increasing a bit but are still below the pre-pandemic levels for now,” Michaels said in an email.