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. This pathogen is specific to humans; it has not been shown to infect other animals, while other species of Pneumocystis that parasitize other animals have not been shown to infect humans. 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. The older name Pneumocystis carinii, (which now only applies to the Pneumocystis species that is found in rats), is still in common usage. As a result, Pneumocystis pneumonia (PCP) is also known as Pneumocystis jiroveci pneumonia and (incorrectly) as Pneumocystis carinii pneumonia. Regarding nomenclature, when the name of Pneumocystis pneumonia changed from P. carinii pneumonia to P. jirovecii pneumonia, it was at first felt that it could no longer be referred to as "PCP". However, because the term PCP was already in common usage, it was rationalized that the term PCP could continue to be used, as it stood for PneumoCystis (jirovecii) Pneumonia.