Pneumocystis pneumonia is a lung infection that occurs primarily in people with weakened immune systems especially people who are HIV-positive. The disease agent is an organism whose biological classification is still uncertain. Pneumocystis carinii was originally thought to be a one-celled organism (a protozoan), but more recent research suggests that it is a fungus. Although its life cycle is known to have three stages, its method of reproduction is not yet completely understood. The complete name of the disease is Pneumocystis carinii pneumonia (PCP) and is also sometimes called pneumocystosis.

Pneumonia as a general term refers to a severe lung inflammation. In pneumocystis pneumonia, this inflammation is caused by the growth of Pneumocystis carinii, a fungus-like organism that is widespread in the environment. PCP is ordinarily a rare disease, affecting only people with weakened immune systems. Many of these people are patients receiving drugs for organ...