Pneumocystis jirovecii, formerly known as Pneumocystis carinii f. sp. hominis, is the causative agent of Pneumocystis pneumonia (PCP). PCP is known to be one of the most frequent and severe opportunistic infections in immunocompromised individuals and the organism was detected in the lungs of a large percentage of the population that did not have underlying disease. Serologic studies indicate that antibodies against the organism can be detected in most children early in life, indicating frequent exposure to the organism. It has been hypothesized that disease in immunocompromised individuals is a reactivation of latent infections acquired during childhood. However, animal and human studies have shown that the organism is eliminated after infection and that the latency is very limited. Healthy individuals can acquire the organism through airborne droplets.

Diagnosis of Pneumocystis jirovecii was based on clinical findings that evaluate clinical history of Pneumocystis jirovecii carriage, chest X-rays, HIV infection status and IFA findings in the lab. Detection methods have improved and detection of lower numbers of the organism are now possible through the use of Polymeraes Chain Reaction (PCR) and nested PCR.