AbstractPneumocystis cariniiwas first described in the lungs of guinea pigs during studies of experimental American trypanosomiasis by Chagas in 1909.1The organism was thought to represent a variant in the sexual life cypcle ofTrypanosoma cruzi. Carini found the organism in trypanosome-infected rats, but it was not until 1912 that the organisms were recognized as belonging to a new genus and assigned the name ofPneumocystis cariniiwhen Delanöes identified identical forms in the lungs of rats which had not been infected with trypanosomes.2