Pneumocystis jirovecii, formerly known as Pneumocystis carinii, is the causative agent of Pneumocystis pneumonia (PCP), a disease almost exclusively seen in the setting of immunosuppression. It is most commonly seen in HIV-infected individuals with a CD4 cell count of < 200/mm3 and in patients on high-dose glucocorticoids or on immunosuppressive agents related to cancer chemotherapy or organ transplantation.

The distribution of the organism is worldwide, and most healthy children are exposed to it at an early age. However, disease manifestations are seen in the setting of immunosuppression. The risk of disease in the HIV-infected population rises dramatically at CD4 cell counts < 200/mm³, in the absence of appropriate prophylaxis. The presence of oral thrush and fevers are also independently associated with an increased risk, irrespective of the CD4 cell count.

Typical onset in patients with HIV is insidious with a fever and dry cough and progressive shortness of breath. Chest pain and hemoptysis may rarely be present. The symptoms may be more acute in onset in non-HIV-infected patients. Infection is often asymptomatic in lung transplant recipients.

Physical examination usually reveals fever, tachycardia, and tachypnea. The breath sounds are often normal, but in up to one third of adults, rales are present. Impaired oxygenation (arterial blood gases) is a common finding, with varying degrees of hypoxemia and elevated alveolar-arterial oxygen gradient. Although nonspecific, serum LDH is often highly elevated and declines with successful therapy.