Pneumocystis

This is an infection of the lung caused by Pneumocystis carinii. The organism is a common cause of fatal pneumonia in AIDS patients. P. carinii is a commensal of many wild and domestic animals and evidence suggests that human infection is commonly derived from dogs.

An intracellular parasite, with a life cycle of trophozoite and cyst, it was formerly considered to be a protozoan. However, comparison of DNA and RNA sequences have established that it is one of the group of ustomycetous red yeast fungi. The cysts contain 8 nuclei which can be seen in smears of pulmonary aspirates.