Infection with the organism Pneumocystis carinii, usually occurring only in infants or immunosuppressed persons (e.g., those with acquired immune deficiency syndrome); usually involves the lungs (pneumonia) and is characterized by fever, cough, rapid breathing, and cyanosis. This form of pneumonia may be fatal. Current treatment includes pentamidine and sulfa drugs (e.g., Bactrim); other agents are being tried experimentally. The responsible microbe has been thought to be a protozoan, though some studies suggest that it may be a fungus.