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Pneumonia (nu-MO-ne-ah) is swelling (inflammation) of one or both lungs that is usually caused by an infection. Many different germs can cause pneumonia, including bacteria, viruses, and fungi. When you breathe in these germs, they can settle in the air sacs (alveoli) of your lungs. Deep in your lungs, the germs may grow and overcome your body's normal defenses.

After the lungs become infected, the air sacs (alveoli) in the lungs fill with pus and mucus. This swelling (inflammation) of the air sacs makes them less stretchy and keeps oxygen from properly reaching your blood stream.

As you work harder to breathe and give your body oxygen, you can feel short of breath. The swelling also causes many of the other symptoms of pneumonia like cough, fever, and chest pain.

Pneumonia can be life-threatening. It's a leading cause of death and hospitalization in seniors and in people with long-term (chronic) diseases. The good news is that there are many things you can do to lower your risk of getting pneumonia.