The Centers for Disease Control and Prevention has warned of an unusual increase in bacterial infections caused by Mycoplasma pneumoniae. But what exactly is it and why is it concerning?

According to the alert from the CDC, infections from the bacteria have been rising since the spring, peaking in August and remaining high ever since. But while the rise itself is larger than previous years, what makes this spike unique is who the cases are impacting most.

"The proportion of patients discharged from emergency departments with a diagnosis of M. pneumoniae-associated pneumonia or acute bronchitis has been increasing over the past six months, peaking in late August," the CDC said in an alert on Oct. 18.

Experts said the rise started nationally in the spring, and began in June in Illinois.

"It's really dramatic," Dr. Sharon Welbel, the director of Hospital Epidemiology and Infection Control for Cook County Health, told NBC Chicago. "A dramatic increase in mycoplasma pneumoniae from 2023 at this time of year to what's happening this year ... but also a really dramatic increase in the 2 to 4 year olds. So that's new."

The latest trend marks a shift from previous years as cases typically are seen in school-aged children and adolescents, the CDC noted.

"M. pneumoniae infections can occur at any age, but they most often occur among children ages 5–17 years and young adults," the CDC reported.

But discharge data from March 31 through Oct. 5 of this year showed an increase "among all groups" in the U.S., particularly children between the ages of 2 and 4 years old. In that age group, cases climbed from 1% to 7.2%. Children ages 5-17 years saw cases jump from 3.6% to 7.4% in the same time frame.

"The increase in children ages 2–4 years is notable because M. pneumoniae historically hasn't been recognized as a leading cause of pneumonia in this age group," the CDC reported.

Overall, infections from Mycoplasma pneumoniae aren't uncommon, with an estimated 2 million reported each year in the U.S. The actual numbers remain unknown, however, as there is no national reporting or surveillance system tracking such infections.

Cases typically rise in summer and early fall, according to the agency.

Here's what to know:

What is mycoplasma pneumoniae?

Mycoplasma pneumoniae is considered a common cause of "mild respiratory illness." The bacteria can infect various parts of the body, though it is typically known for causing respiratory tract infections, which can "damage the lining of the respiratory tract, including the throat, windpipe, and lungs."

What is 'walking pneumonia'?

While the CDC said infections are generally mild and may only appear as a chest cold, they could also lead to pneumonia, but not your typical pneumonia.

"When an M. pneumoniae infection progresses to pneumonia, it's typically a less severe form of bacterial pneumonia commonly referred to as 'walking pneumonia,'" the CDC reported.

Such cases are used to describe people with pneumonia who "seem better than expected for someone with a lung infection."

"With mild symptoms, people may not stay home or in bed. Thus, the phrase 'walking pneumonia' was coined," the CDC reported.

What are the symptoms to watch for?

Symptoms for infection can vary depending on age and level of infection. Some people may have no symptoms at all, while others could experience chest cold or pneumonia-like symptoms. The symptoms for children are also different.

Typically, it can take one to four weeks for symptoms to appear after someone has been exposed to the bacteria. Symptoms can then last for several weeks.

Here's what the CDC says to watch for:

Common symptoms include:

• Feeling tired
• Fever
• Headache
• Slowly worsening cough
• Sore throat

Symptoms in children

Instead of chest cold symptoms, children younger than 5 years old may have:

• Diarrhea
• Sneezing
• Sore throat
• Stuffy or runny nose
• Vomiting
• Watery eyes
• Wheezing

Pneumonia or 'walking pneumonia' symptoms

Common symptoms include:

• Cough
• Feeling tired
• Fever and chills
• Shortness of breath

How does it spread?

According to the CDC, the bacteria spread by inhaling respiratory droplets from an infected person. But infections typically have a long incubation period, with symptoms not starting for one to four weeks after exposure.

The bacteria can also remain in the respiratory tract for months after infection.

"Most people who spend a short amount of time with someone who is sick with M. pneumoniae don't get infected. However, people who spend a lot of time with someone who is sick with M. pneumoniae are at increased risk," the CDC reported.

How is it treated?

According to Welbel, mycoplasma pneumoniae is often considered an "atypical bacteria," which can mean some antibiotics won't work on it.

"We often call it an atypical bacteria because it does not have a cell wall," she said. "And that's just important, because many antibiotics work by destroying the cell wall, and so we have to know what type of antibiotics to use for this."

The CDC similarly noted that "M. pneumoniae bacteria are naturally resistant to certain antibiotics that are commonly used to treat other kinds of bacterial pneumonia."

The CDC noted that while many will recover on their own, over-the-counter medicines can also be helpful.