any protozoan of the genus Pneumocystis, esp P. carinii, which is a cause of pneumonia in people whose immune defences have been lowered by drugs or a disease
pneumocystis in Medicine
pneumocystis pneu·mo·cys·tis (nōō'mə-sĭs'tĭs, nyōō'-) n. A severe lung infection caused by the parasitic protozoan Pneumocystis pneumonia and affecting primarily individuals with an immunodeficiency disease, such as AIDS.