DENVER — Colorado has experienced a surge in walking pneumonia this fall, most notably among very young children who aren’t usually as susceptible to the illness, according to health officials.

Bacterial infections that cause the mild lung infection reemerged nationwide this year for the first time since the start of the pandemic, peaking in August after increasing sharply since the spring, according to the Centers for Disease Control and Prevention.

In Colorado, walking pneumonia cases peaked a little later, in September, but the illness is expected to continue spreading at a high rate into the new year, said Dr. Kevin Messacar, an infectious disease specialist at Children’s Hospital Colorado.

Doctors also are discovering cases more frequently in younger children, aged 2 to 4, despite the infections being historically more common in school-aged children, he said.

“What we are seeing now is a several-fold increase than what we typically see,” Messacar said.

Mycoplasma pneumoniae bacteria, which causes walking pneumonia, spreads via respiratory droplets when a person sneezes or coughs. Mycoplasma pneumoniae infections don’t spread as quickly as other illnesses, such as the flu, and can takes weeks to make their way through a whole family, Messacar said.

The spread of the bacteria can be prevented through handwashing and a person covering their mouth and nose when they cough or sneeze, according to the CDC.

Walking pneumonia is typically milder than other forms of pneumonia, and symptoms include fever, chills, sore throat and rashes. The hallmark of the illness is “a nagging, persistent cough” that settles into the lungs and can last as long as four weeks, Messacar said.

The illness is named walking pneumonia because people with the illness may not stay in bed or even at home because their symptoms are milder, according to the CDC. But more serious cases can lead to hospitalization, Messacar said.

Health officials said they’re unsure exactly how prevalent walking pneumonia is in Colorado — including how many children have been hospitalized for the illness — because testing is not common. Recently, a 3-year-old in Colorado was hospitalized with walking pneumonia, CBS Colorado reported.

The Colorado Department of Public Health and Environment hasn’t received any reports of walking pneumonia outbreaks this fall, but has noted an increase in patients visiting hospital emergency departments with such infections, spokeswoman Kayla Glad said in a statement.

The CDC estimates that 2 million people get sick with walking pneumonia each year across the U.S.

Outbreaks of walking pneumonia historically run on three- to seven-year cycles, but cases dropped during the pandemic as people wore masks and social-distanced to curb the spread of COVID-19. It didn’t reemerge until 2023, unlike the flu or respiratory syncytial virus, or RSV, both of which returned sooner, Messacar said.

Walking pneumonia has notably increased in young children, with infections among kids aged 2 to 4 jumping from 1% of all cases nationwide on March 31 to 7.2% of cases on Oct. 5. This is unusual because Mycoplasma pneumoniae isn’t normally a leading cause of pneumonia in young children, according to the CDC.

“I don’t think we have an answer for exactly why that is,” Messacar said, adding that, generally, when there hasn’t been an outbreak for a long period, then more of the population becomes susceptible to the bacteria because they haven’t built up immunity.

“I do think that this is part of the same phenomena that we saw with RSV and influenza,” which came back strong after initially being suppressed during the early phase of the pandemic, he said.

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