The identity of Pneumocystis carinii: not a single protozoan, but a diverse group of exotic fungi.

Pneumocystosis is an AIDS-associated pneumonia that is characterized by the accumulation of very large numbers of a eucaryotic single-celled organism called Pneumocystis carinii, which has not been cultured. Pneumocystosis also occurs in many other mammalian species, and the rubric P. carinii is currently used to refer to organisms associated with this disease in all host species. This article reviews molecular genetic data establishing that P. carinii is a fungus, that P. carinii in people is not the same organism as P. carinii in rats, and that there may be more than one species of P. carinii, capable of infecting an individual patient. The implications of such genetic diversity for understanding and combating pneumocystosis are discussed.