The history of the discovery of Pneumocystis carinii as the cause of interstitial plasma cell pneumonia of infants and of opportunistic infection with this parasite in the lungs of immuno-suppressed or severely debilitated adults and children is summarized with an extensive bibliography of the world literature through 1959. The problem of incriminating a nearly ubiquitous saprophyte as the cause of a disease and of elucidating the factors responsible for its causing pathogenesis in certain individuals is reviewed with special reference to slow and latent virus infections. Although formerly there has been much discussion of other possible etiologies of the respiratory syndromes in premature and debilitated infants and in children and adults who are immunodeficient or immunosuppressed in whose lungs the parasite is found, there seems no longer to be any room for doubt that P. carinii is itself the cause of these respiratory disease. The need of determining the genetic constitutions and the specific immunologic deficiencies that render some persons victims to this usually innocuous saprophyte is compelling, and the in vitro cultivation of the organism may aid considerably in unraveling these problems.