Pneumocystis pneumonia (PCP) : Pneumocystis pneumonia or pneumocystosis is a form of pneumonia, caused by the yeast-like fungus Pneumocystis jirovecii. Pneumocystis carinii pneumonia is the most common opportunistic infection in persons with HIV infection. Pneumocystis is commonly found in the lungs of healthy people, but, being a source of opportunistic infection, it can cause a lung infection in people with a weak immune system. Pneumocystis pneumonia is especially seen in people with HIV/AIDS and the use of medications that affect the immune system. Antibiotics are primarily recommended for treatment of mild, moderate, or severe PCP. Trimethoprim-sulfamethoxazole has been shown to be as effective as intravenous pentamidine and more effective than other alternative treatment regimens. Corticosteroids are used as adjunctive initial therapy only in patients with HIV infection who have severe PCP.