PCP is a pneumonia caused by the fungal organism Pneumocystis carinii, which is widespread in the environment, and is not a pathogen (does not cause illness) in healthy individuals. However, in individuals with weakened immune systems due to cancer, HIV/AIDS, solid organ and/or bone marrow transplantation, as well as individuals receiving chronic corticosteroids or other medications that affect the immune system, Pneumocystis carinii may lead to a lung infection. Individuals with advanced AIDS are of particular interest, since PCP was a relatively rare infection prior to the AIDS epidemic. Before the use of preventive antibiotics for PCP, up to 70% of individuals in the U.S. with advanced AIDS would develop PCP. PCP in the setting of AIDS usually develops slowly and is less severe -- several weeks of cough, fevers, and progressive shortness of breath, especially with exertion. Individuals with PCP who do not have AIDS usually get sick faster and are more acutely ill.
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