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.
Learn more at:

http://www.nhlbi.nih.gov/health/health-topics/topics/pnu