Pneumocystis produces respiratory infection in immunocompromised individuals of several species of mammals, including humans. Each mammalian species has its own specific Pneumocystis species, which does not cross-infect other mammals. The species infecting humans has now been renamed P. jerovici, since P. carinii is reserved for one of two species infecting rats. Long believed to be a protozoan, Pneumocystis is now classified as an Archiascomycetous fungus. This is based on new molecular taxonomic techniques using DNA sequence analysis of srRNA genes. Only two of about 140 copies of the gene that exist in Pneumocystis were used for sequencing, so the evidence is not conclusive; however, it is supported by morphological evidence such as fungus-specific nucleus-associated organelles for cell division. There is also ultrastructural evidence of meiotic division and sexual conjugation. Clinically, several lines of evidence suggest the improbability of latent infection. Adult infections appear to be new infections, a fact that invites a new perspective on prevention.