Pneumonia is most often caused by bacteria. About a third of the cases of pneumonia in the U.S. come from viruses.
The bacteria or virus gets trapped deep in the lungs, in the small air sacs known as alveoli. The immune system starts fighting the germs and causes inflammation.
The symptoms of pneumonia include shortness of breath and coughing, plus all the other symptoms that usually go with an infection, such as:
- fever
- chills
- body aches
- fatigue
Sometimes pneumonia follows another infection, like a cold or the flu. However, pneumonia can come on all by itself, without any other illness.
Some people are more at risk for pneumonia. The elderly and those with weakened immune systems are the most vulnerable.
A small number of pneumonia cases come from fungal infections. A common fungus is responsible for a condition called pneumocystis jiroveci or pneumocystis carinii pneumonia (PCP). Generally, PCP only affects people with weak immune systems, like patients with HIV/AIDS or people who've had organ transplants.
Source:
"Pneumonia." NIH: National Heart Lung and Blood Institute. Last updated: Aug 2008. Accessed 3 Sep 2010
