The U.S. may or may not be headed for a “tripledemic” scenario like it saw last year—one in which three main pathogens drive up illnesses, hospitalizations, and general misery.

The term tripledemic was a misnomer, experts say, as far more than three viruses—COVID, flu, and RSV—were responsible for the surge in sickness. So were influenza-like illnesses like rhinoviruses, respiratory adenoviruses, and other common coronaviruses, to name a few. Certain bacterial infections, like mycoplasma pneumonia and group A streptococcus, can also cause winter-time infirmity. 

All of these conditions—and more—can cause pneumonia. That’s why it’s the most common cause of pediatric hospital admission in the U.S., and the most common cause of adult pediatric hospital admission (second only to childbirth). 

We’ll see plenty more pneumonia this year, whether we have a tripledemic, syndemic, or even a singular epidemic. Here’s what you need to know to keep safe, according to experts.

What is pneumonia?

Pneumonia is an infection of one or both lungs. It 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, Dr. Carrie Horn, chief medical officer at leading U.S. respiratory hospital National Jewish Health in Denver and a hospitalist, recently told Fortune.

What are the signs and symptoms of pneumonia?

Symptoms can range from mild to serious, according to the U.S. National Heart, Lung, and Blood Institute, and may include:

cough without mucus

fever

chills

trouble breathing

chest pain when you breathe or cough

low oxygen levels in your blood

Other, less common symptoms can include:

headache

muscle pain

extreme tiredness

nausea 

committing

diarrhea

Older adults and those with weakened immune systems may have atypical symptoms like:

lower-than-normal temperature instead of a fever

sudden feeling of weakness

sudden feeling of confusion

Babies may also have particular symptoms, which can include:

vomiting

fever

cough

restless appearance

fatigue

Babies may also show specific signs of breathing problems, including:

bluish tone on skin or lips

grunting

pulling inward of muscles between ribs while breathing

rapid breathing

widening of nostrils during breathingWhat are emergency signs of pneumonia?

Severe signs that should cause people to seek medical attention include:

chest pain

severe trouble breathing

low oxygen levels, which may present as a bluish or gray color to the fingernails or lips

needing more oxygen than usual

a fever lasting longer than just a day or two

coughing up blood

low blood pressure

a cough that just won’t go away

passing out

a fever over 100.4°F for children 6 months and younger, or above 102°F for children older than 6 months

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

What is ‘white lung pneumonia’?

In a normal, healthy person, “lung tissue is so delicate that an X-ray passes right through, and the lung fields appear black,” Dr. William Schaffner, professor of infectious diseases at Vanderbilt University in Nashville, tells Fortune.

In those with pneumonia, however, lung tissue becomes bulkier “with all those inflammatory cells that come in,” which makes the tissue more dense, he adds.

In a lung X-ray of a person with pneumonia, “the pneumonia part appears white,” he says—hence the non-medical term “white lung pneumonia” currently trending online. 

“I think that’s what the name is in reference to,” he says, “although, unfortunately, it sounds very scary.”

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., recently told 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,” Cowl says. What’s more, if the bacteria causing pneumonia spread to the bloodstream, sepsis could result—a life-threatening condition prompted by an infection that triggers a chain reaction throughout the 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. What’s more, RSV vaccines are now available for infants at high risk of severe disease, pregnant women from 32-36 gestational weeks, and adults 60 years of age and older.

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.

Original: Fortune | FORTUNE: Pneumonia signs and symptoms | Fortune Well