Fatal Pneumocystis carinii pneumonia has been reported with increasing frequency in the United States during the past decade. Death is apparently related to impaired gas exchange resulting from masses of organisms filling pulmonary air spaces. Pneumocystis pneumonia, first described well in 1938 in debilitated European infants, was called "plasma cell interstitial pneumonia" prior to the recognition of masses of characteristic organisms in lung sections of such cases.
Pneumocystis carinii has not been cultured, and occurs naturally in lungs of apparently healthy laboratory animals, thus complicating the fulfillment of Koch's postulates. For similar reasons the taxonomic position of the organism is unsettled, although the weight of evidence suggests that it is a protozoan. The source of human infection is unknown. Pneumocystis carinii does not occur commonly in sputum of healthy individuals as does Candida, for example. Hospital personnel attending affected infants in countries where P carinii pneumonia is common have a