Pneumocystis carinii pneumonia (PCP) is the former name of Pneumocystis jiroveci pneumonia (PJP). It is the most common opportunistic infection which occurs in persons infected with human immunodeficiency virus (HIV). P jiroveci is one of several organisms that are known to cause opportunistic infections in HIV infected patients and PJP does not respond to antifungal treatments. Pneumocystis is a form of atypical pneumonia. In patients with early disease who are not visibly immunocompromised, laboratory diagnosis of PCP is critical. In patients with pneumonia and HIV infection, PCP is the primary consideration. P jiroveci cannot be cultured in vitro, thus the laboratory detection involves using a PCR (polymerase chain reaction) assay.