More than one out of every 10 deaths in the U.S. during the third week of 2018 was from influenza or pneumonia, according to data released Friday by the Centers for Disease Control and Prevention (CDC).

Pneumonia, or inflammation of the lungs caused by an infection, is a common flu complication that can result in hospitalization and even death.

CDC acting director Anne Schuchat, M.D., said in a media briefing on Feb. 9 that with flu-related hospitalization rates approaching record numbers, flu and pneumonia deaths are expected to continue to rise over the next few weeks. 

How Flu Leads to Pneumonia

Flu can cause pneumonia in two ways. First, the influenza virus itself can infect the lungs, causing what’s called viral pneumonia. Second, flu weakens the immune system and damages the lining of the respiratory tract, which can make it easier for certain bacteria to infect the lungs and cause bacterial pneumonia.

More on flu season

Experts aren’t sure exactly what percentage of people who get flu will go on to develop pneumonia, partly because the symptoms of the two can be similar, and there isn’t a definitive lab test for pneumonia. Estimates of the number of flu patients who also develop a bacterial infection, for example, range from as low as 2 percent to as high as 65 percent, according to one analysis.

But the people most at risk for a serious complication from flu include young children, pregnant women, and older adults, whose immune systems tend to be weaker, as well as people with certain underlying health conditions, such as lung disease, heart disease, blood disorders (such as sickle cell anemia), diabetes, immune-suppressing diseases (such as HIV), or conditions like cancer that require immunosuppressive drugs.

Here, the steps to prevent and diagnose pneumonia. 

Get a Flu Shot

The best way to protect against flu-related pneumonia is to avoid the flu itself. The CDC is still recommending people get a flu shot if they haven’t already, “even this late in the season,” Schuchat says.

Although the vaccine doesn’t perfectly protect against flu, it does reduce your chances of catching the illness.

Also, evidence suggests that even if you get the flu, the shot may lessen the severity of the illness. That, in turn, means you have a smaller chance of developing a secondary bacterial infection, says Todd Rice, M.D., medical director of the medical ICU at Vanderbilt University Medical Center.

Consider a Pneumococcal Vaccination

Pneumococcal bacteria, which cause some cases of sepsis and meningitis as well as pneumonia, may be responsible for about a third of bacterial infections related to flu.

There are currently two vaccines that are recommended for some high-risk groups as protection against all kinds of pneumococcal disease, including pneumonia.

The first, Prevnar 13, protects against 13 different strains of the bacteria, while the second, Pneumovax 23, protects against 23 strains (there's some overlap between the shots).

Healthy adults under 65 probably don’t need them, however, because they’re not at a high risk for pneumococcal disease. Here’s who does need a pneumococcal vaccine, according to CDC guidelines:

Children under 2 receive Prevnar as part of their regular vaccination schedule, something that started back in 2000. They should get a dose at 2, 4, 6, and 12 to 15 months old.

Adults 65 and older should receive one dose each of Prevnar and Pneumovax, one year apart. The CDC recommends getting Prevnar first, as soon as you turn 65. (If you got Pneumovax first, wait a year and get Prevnar.) If you didn’t get it when you turned 65, ask your doctor whether you can get the shot now; there’s no upper age limit for receiving either vaccine.

People with underlying health conditions who are between the ages of 2 and 64 should consult their doctor about which vaccines they should receive and when. Recommendations vary depending on your age and disease, but people who are more vulnerable to a flu complication because of a pre-existing health problem may benefit from the added protection of the vaccine.

Note, however, that these vaccines aren’t perfect, says Elaine Tuomanen, M.D., pediatric infectious disease specialist at St. Jude Children's Research Hospital who studies pneumococcal vaccines. There are almost 100 different known strains of pneumococcal bacteria—many more than are included in the vaccines. But it’s still important for vulnerable people to receive them, to reduce their chances of serious illness.

Reduce Your Exposure to Bacteria

It’s a good idea to stay home if you have the flu so that you avoid spreading the infection to others. But staying home until you feel well again can also help you protect against pneumonia, Tuomanen says. That’s because the more you’re out and about in the community, the greater the odds that you'll expose yourself to pneumococcal bacteria by getting it on your hands and touching your face.

Exposure to this pathogen when your immune system is already weakened by flu can make it easier for a bacterial infection to take hold. And though you should always be diligent about washing your hands, this is why it’s especially important to wash them frequently when you’re recovering from the flu.

Because of the damage it causes to your lungs, smoking cigarettes can also make you more likely to develop pneumonia.

Know the Signs, and Act Fast

The signs of flu-related bacterial pneumonia are distinctive—you’ll usually start recovering from the flu and feeling better, before suddenly developing a new fever and getting worse again. If that happens, you should see your doctor right away or visit an emergency room, according to the CDC. 

Bacterial pneumonia can often be treated with antibiotics.

Delaying treatment of bacterial pneumonia can raise your risk of becoming seriously ill, so you shouldn’t wait if you unexpectedly take a turn for the worse.

You should also visit the emergency room if you start to have trouble breathing—another emergency sign of viral or bacterial pneumonia.

Viral pneumonia can sometimes be harder to detect, because it probably won’t include a telltale period of temporary improvement. You’ll have a typical case of flu at first, but instead of getting better after a few days, you’ll continue to feel worse, your fever won’t break, and you may have trouble breathing, says Almea Matanock, M.D., who studies pneumococcal disease at the CDC.

Anyone who is in doubt about whether he has pneumonia should call his doctor immediately and ask whether he should come in to be evaluated.

For children under 5, adults over 65, and anyone with an underlying health problem, it's especially important to call the doctor as soon as symptoms appear. People in these groups may especially benefit from antiviral medications, such as Tamiflu (oseltamivir).

Antivirals may lessen the length and severity of illness, but they work best when started within 48 hours of the onset of symptoms. A doctor still might decide to prescribe them to anyone experiencing a severe flu complication, however, even outside the 48 hour window, Rice says.