Pneumonia (nu-MO-ne-ah) is an infection in one or both of the lungs. Many germs—such as bacteria, viruses, and fungi—can cause pneumonia.
The infection inflames your lungs’ air sacs, which are called alveoli (al-VEE-uhl-eye). The air sacs may fill up with fluid or pus, causing symptoms such as a cough with phlegm (a slimy substance), fever, chills, and trouble breathing.
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