ABSTRACT
I. Introduction The history of Pneumocystis carinii opens with a series of seemingly trivial events beginning around the turn of the twentieth century [1]. Its emergence from an obscure, insignificant infection of rodents to a major, life-threatening infectious disease of humans over only a few decades was due to changes in the human host, not the organism. Remarkable in modern medicine is the fact that the discovery of P. carinii was accomplished in great part by simple studies of human and animal lung tissues with the compound microscope —as were the elucidation of its structure, pathogenesis, epidemiology, and host response mechanisms and also the development of therapeutic and prophylactic agents against it. Only in the last decade and a half have the powerful tools of molecular biology been applied to P. carinii.
