Pneumocystis (carinii) jiroveci pneumonia can occur in immunocompromised individuals, especially hematopoietic stem and solid organ transplant recipients and those receiving immunosuppressive agents, and is the most common opportunistic infection in persons with advanced human immunodeficiency virus (HIV) infection. The Pneumocystis genus was initially mistaken as a trypanosome and later as a protozoan. Genetic analysis identified the organism as a unicellular fungus. Pneumocystis jiroveci is the species responsible for human infections. A slow indolent time course with symptoms of pneumonia progressing over weeks to months is characteristic in HIV-infected patients. Fulminant respiratory failure associated with fever and dry cough is typical in non-HIV-infected patients. Definitive diagnosis relies on histopathological testing of sputum, induced or sampled by fiberoptic bronchoscopy with bronchoalveolar lavage. The first-line drug for treatment and prevention is trimethoprim-sulfamethoxazole.