Pneumocystis carinii is an endogenous liposomally modified mitochondrion

Pneumocystis carinii was first described over 70 years ago but its taxonomy is still uncertain. Doubts have been expressed as to whether Pneumocystis carinii is an organism at all; but no coherent alternative has been proposed to account for the pathogenesis of pneumocystosis. Consideration of recent observations on Pneumocystis, liposomes, pulmonary surfactant and mitochondria leads to the hypothesis that Pneumocystis carinii is derived from liposomal change in the type II pneumocyte mitochondria that are known to be secreted into the alveoli under a variety of stressful stimuli. This hypothesis is considered to be more consistent with the facts than the hypothesis that Pneumocystis carinii is a protozoon or a fungus or any other kind of exogenous microorganism.