This form of pneumonia is caused by Pneumocystis carinii, a protozoan of uncertain taxonomic status in the class Sporozoa. An extracellular parasite of the lungs of humans, dogs, rodents, and other mammals, the organism occurs worldwide. It appears to be of low virulence and almost never causes disease except in weak or immunosuppressed individuals. P. carinii was discovered in guinea pigs in 1909, but human disease was first recognized in the 1940s in malnourished and premature infants. Patients suffering from leukemia, Hodgkin’s disease, and other immunosuppressive diseases, or organ transplant recipients and other patients whose treatment requires suppression of the immune system, are also vulnerable to infection. In the early 1980s, pneumocystis pneumonia achieved prominence as the most common opportunistic infection afflicting patients with acquired immune deficiency syndrome (AIDS). Over half of all AIDS victims suffer from this form of pneumonia, and it frequently is the proximate cause of death.

Transmission is usually by airborne droplets, although transplacental passage resulting in fetal death has been reported. Latent infection may be common, with clinical disease and droplet transmission developing only in weakened hosts. The parasite damages the alveolar walls and induces an abundant foamy exudate and fibrosis. Death results from asphyxiation due to the exudate. Although initial response to chemical therapy is common, treatment is difficult because of side effects of the drugs and the debilitated state of the patients.