Author: Jan Scholten
Clades: Pneumocystidaceae, Pneumocystidales, Pneumocystidomycetes, Taphrinomycotina, Ascomycota, Fungi, Plants.
Mycology: extracellular parasite. All stages are found in lungs and because they cannot be cultured ex-vivo, direct observation of living Pneumocystis is difficult. The trophozoite stage is thought to be equivalent to the so-called vegetative state of other species (such as Schizosaccharomyces pombe) which, like Pneumocystis belong to the Taphrinomycotina branch of the fungal kingdom. The Trophozoite stage is single-celled and appears amoeboid (multilobed) and closely associated with host cells. Globular cysts eventually form that have a thicker wall. Within these ascus-like cysts, eight spores form which are released through rupture of the cyst wall. The cysts often collapse forming crescent-shaped bodies visible in stained tissue. It is not known for certain if meiosis takes place within the cysts, or what the genetic status is of the various cell types.
Pneumocystis pneumonia (PCP) or pneumocystosis is a form of pneumonia, caused by the yeast-like fungus (which had previously been erroneously classified as a protozoan) Pneumocystis jirovecii. 
Pneumocystis is commonly found in the lungs of healthy people, but, being a source of opportunistic infection, it can cause a lung infection in people with a weak immune system. Pneumocystis pneumonia is especially seen in people with cancer, HIV/AIDS and the use of medications that affect the immune system.