Pneumocystis pneumonia (PCP) is a form of pneumonia. It affects people who have a weakened immune system. PCP is the most common serious infection among people with AIDS.

The Lungs (Cut-away View)
Nucleus factsheet image
Copyright © Nucleus Medical Media, Inc.

PCP is caused by a fungus. Most believe that the fungus is spread in the air but it is not clear if the fungus lives in soil or elsewhere. In healthy people, the fungus can exist in the lungs without causing pneumonia. However, in people who have a weakened immune system, the fungus can spread and cause a lung infection.