Pneumocystis jiroveci.

remains an important fungal pathogen in a broad range of immunocompromised hosts. The natural reservoir of infection remains unknown. Pneumonia (PJP) develops via airborne transmission or reactivation of inadequately treated infection. Nosocomial clusters of infection have been described among immunocompromised hosts. Subclinical infection or colonization may occur. Pneumocystis pneumonia occurs most often within 6 months of organ transplantation and with intensified or prolonged immunosuppression, notably with corticosteroids. Infection is also common during neutropenia and low-lymphocyte counts, with hypogammaglobulinemia, and following cytomegalovirus (CMV) infection. The clinical presentation generally includes fever, dyspnea with hypoxemia, and nonproductive cough. Chest radiographic patterns are best visualized by computed tomography (CT) scan with diffuse interstitial processes. Laboratory examination reveals hypoxemia, elevated serum lactic dehydrogenase levels, and elevated serum (1→3) β-D-glucan assays. Specific diagnosis is achieved using respiratory specimens with direct immunofluorescent staining; invasive procedures may be required and are important to avoid unnecessary therapies. Quantitative nucleic acid amplification is a useful adjunct to diagnosis but may be overly sensitive. Trimethoprim-sulfamethoxazole (TMP-SMX) remains the drug of choice for therapy; drug allergy should be documented before resorting to alternative therapies. Adjunctive corticosteroids may be useful early in the clinical course; aggressive reductions in immunosuppression may provoke immune reconstitution syndromes. Pneumocystis pneumonia (PJP) prophylaxis is recommended and effective for immunocompromised individuals in the most commonly affected risk groups.