The five-time Olympic medalist is fighting for her life in a Texas hospital, according to the Associated Press, with what family members describe as a “very rare form of pneumonia.”

The respiratory illness most commonly affects the young, babies and toddlers, and the elderly, ages 65 and older. But it still poses a risk to the relatively young like Retton, who is 55, experts tell Fortune.

“Severe pneumonia—meaning that someone is sick enough to need oxygen, be admitted to the hospital, or potentially die from the infection—is relatively rare in healthy people in their midlife age range,” Dr. Carrie Horn, chief medical officer at leading U.S. respiratory hospital National Jewish Health in Denver and a hospitalist, tells Fortune. 

Still, “it can happen, and we don’t always know why it happens to some people and not others,” she adds. “Chronic medical conditions definitely increase the risk of severe pneumonia and complications in that midlife group.”

Here’s what you need to know to protect yourself from the potentially deadly condition, which results in nearly 1.5 million emergency department visits annually in the U.S.

What is pneumonia?

Pneumonia is an infection of one or both lungs that causes air sacs to fill up with fluid or pus, according to the U.S. National Institutes of Health. It can be caused by bacteria, viruses, fungi, or parasites, and its symptoms can range from mild—cough with or without mucus, fever, and chills—to serious, like trouble breathing.

The most common causes of pneumonia are bacteria and viruses, Horn says. Among the most concerning is pneumonia caused by Streptococcus pneumoniae, bacteria that can “spread outside of the lungs into other areas of the body, like the bloodstream and the brain.” 

“Meningitis from Streptococcus pneumoniae is very serious, and one of the reasons for the vaccine in high-risk groups,” she says. “The vaccine isn’t just aimed at preventing pneumonia, but also preventing an infection from spreading.”

How is pneumonia diagnosed?

In those suspected to have the condition, a health care provider will perform a physical exam and order tests like a chest X-ray. Such tests will help to determine what type of pneumonia they have and how to treat it.

What is the treatment for pneumonia?

Depending on the type of pneumonia, it may include an antibiotic, antiviral, or antifungal medication. If the pneumonia is severe, IV antibiotics and oxygen, administered in a hospital setting, may be required. Very serious pneumonia may require ventilation, a machine that breathes for someone, and/or a breathing tube, as well as surgery to remove an infected or damaged part of the lung.

How severe can pneumonia get?

While pneumonia involves the lungs specifically, other organ systems can be affected, Dr. Clayton Cowl, a pulmonologist with the Mayo Clinic in Rochester, Minn., tells Fortune.

“Someone who becomes very ill with pneumonia may have fluid shifts and also have issues with their kidneys or with their liver or with their heart,” he says. What’s more, if bacteria are causing the pneumonia, it can spread to the bloodstream and cause sepsis, a life-threatening condition prompted by an infection that triggers a chain reaction throughout one’s body.

“It can cause a whole kind of downstream effect, a bucket brigade,” he adds. “It’s like the juggler with the sticks spinning the glass plates. If one of those sticks starts wobbling and a plate suddenly falls, all of the others can go in a hurry.”

Is there a vaccine for pneumonia?

Yes, two kinds. But they’re only recommended for those age 4 and younger, and 65 and older. A doctor may recommend that people between those age groups receive one or more of those vaccines if they have a medical condition—like diabetes, heart disease, chronic lung disease, or immunodeficiency—that puts them at higher risk for severe outcomes.

Adults ages 19 to 64 who don’t have a health condition that puts them at higher risk, but who want the pneumonia vaccine, should talk to their doctor about it. If they end up receiving it, they may have to pay cash price, even if they have insurance, Horn says.

What else can I do to protect myself from pneumonia?

Pneumonia vaccines aside, Horn’s best recommendation for all ages is to get the flu and COVID vaccines “to prevent serious lung infections going into the winter season.” Both conditions can lead to pneumonia, in severe cases.

If you have chronic medical conditions, make sure they’re well managed, she advises. “The better controlled they are, the better you will do if you get sick in general.”

Handwashing and staying away from sick individuals can also help prevent infections that may lead to pneumonia.

If I have pneumonia, how do I know it’s time to seek medical help?

Pneumonia, like other respiratory viruses, can cause fevers, chills, and cough. More severe signs that should cause people to seek medical attention include chest pain, severe trouble breathing, low oxygen levels, needing more oxygen than usual, a fever lasting longer than just a day or two, coughing up blood, lips turning blue, low blood pressure, a cough that just won’t go away, and passing out, according to Horn.

“If in doubt, get the evaluation done by a professional,” she adds.