Cases of so-called "walking pneumonia" are spiking across the U.S., the Centers for Disease Control and Prevention (CDC) has alerted.
The highly contagious infection has primarily affected young children, according to the same source.
Between March 31 and Oct. 5 of this year, the percentage of cases grew from 1% to 7.2% among children ages 2 to 4, and from 3.6% to 7.4% among those ages 5 to 17, the agency stated.
Fox News Digital spoke on camera with Dr. Marc Siegel, senior medical analyst for Fox News, about what people should know about this condition.
Also known as "atypical pneumonia," walking pneumonia is a "mild lung infection," as defined by Cleveland Clinic. It tends to feel like a bad cold or the flu.
Typically caused by bacteria, viruses or mold exposure, the condition causes swelling in the airways and fluid in the lungs.
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"Walking pneumonia is less than a full lobar pneumonia, where an entire region of the lungs is whited out from a bacteria or virus," Siegel told Fox News Digital.
"Walking pneumonia generally refers to a patchy pneumonia, where the pathogen isn't affecting one specific region of lung."
Signs of atypical pneumonia may include the following, according to Cleveland Clinic.
The type of cough is a key differentiator between traditional and walking pneumonia, according to Siegel.
"If you have a rip-roaring, traditional bacterial pneumonia, you're going to be coughing up green, brown or dark yellow," he said.
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"But with walking pneumonia, it may be a white mucus … or maybe you don't have a productive cough at all."
This condition also doesn’t cause the high fever associated with full pneumonia, he noted.
As the name implies, you may be able to walk around and go about your daily activities with this less severe form of pneumonia.
"You’re not walking around with a big looming bacterial pneumonia with a high blood cell count and a high fever and chest pain," Siegel said.
Even if it seems milder, however, this type of infection could still require medical attention, the doctor warned.
The condition is easy to miss, as the symptoms are often mistaken for something else, according to Siegel.
"A doctor who is not on the lookout for this could mistake it for a different kind of virus, and not understand that it might actually be an atypical bacterial pneumonia," he told Fox News Digital.
Several different types of bacteria can lead to walking pneumonia, Siegel noted, primarily Mycoplasma.
"This is the season for Mycoplasma," he said. "That's an atypical bacterium that's treatable with certain antibiotics."
Bordetella pertussis, the bacterium that causes whooping cough, can also lead to walking pneumonia.
"There's been a big resurgence of pertussis this year, particularly in very young children," Siegel said.
Legionella, another bacterium that can cause pneumonia, can spread through water or air conditioning units, he warned.
Certain viruses, such as RSV, can also cause pneumonia.
"Although flu doesn't usually cause pneumonia, it can cause secondary pneumonia where you get these infections on top of influenza — the same thing with COVID," Siegel noted.
The resurgence of these pneumonia-causing bacteria and viruses could be a delayed effect of the masking and lockdowns that occurred during the pandemic, the doctor said.
"On top of that, we're not as well-vaccinated as we should be," he said. "And physicians are not on the lookout enough … I'm urging them to be on the lookout for atypical pneumonias."
Siegel said he is most concerned about young children getting these atypical bacteria, because they may not have the ability to fend them off due to not having fully developed lungs.
"I'm also concerned about the elderly, the immunocompromised and those with chronic illness — especially lung illnesses like asthma and emphysema, as they may not have the reserve they need to fight it off," he said.
If a virus is behind the walking pneumonia, it may go away without treatment, Siegel said, but it could be more prolonged.
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"My concern is that we're overlooking atypical bacteria that would go away a heck of a lot faster if we treated them with antibiotics," the doctor said.
"Things like mycoplasma, legionella, pertussis — that's what I'm most concerned about here."
A health care provider can diagnose walking pneumonia by conducting a physical exam, listening to the patient’s lungs, running blood tests and/or performing a chest X-ray, Cleveland Clinic stated.
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Bacterial pneumonias are treated with antibiotics. For viral pneumonias, over-the-counter medications can help relieve symptoms.
Experts also recommend drinking plenty of fluids, getting lots of rest and keeping the airways open by using a humidifier.