Pneumocystosis is an infection by Pneumocystis jirovecii that primarily occurs as a pulmonary infection AIDS patients, with extrapulmonary involvement being uncommon but, if occurring in the skin, presenting most often as nodular growths in the auditory canal. Pneumocystis jirovecii is a yeast-like fungus of the genus Pneumocystis. The causative organism of Pneumocystis pneumonia, it is an important human pathogen, particularly among immunocompromised hosts. Prior to its discovery as a human-specific pathogen, P. jirovecii was known as P. carinii. The complete life-cycles of any of the species of Pneumocystis are not known, but presumably all resemble the others in the genus. The terminology follows zoological terms, rather than mycological terms, reflecting the initial misdetermination as a protozoan parasite. All stages are found in lungs and because they cannot be cultured ex-vivo, direct observation of living Pneumocystis is difficult.