Pneumocystis Jirovecii is a unique yeast-like fungus (previously named Pneumocystis carinii). It has been demonstrated as a predominant cause of intrapulmonary and rarely extra-pulmonary infection and a significant cause of morbidity and mortality in immunocompromised patients with and without AIDS. Pneumocystis jirovecii was classified previously as protozoan endoparasite, and then reclassified as a fungus, based on nucleic acid analysis and biochemical characteristics. No successful cultural method for the organism had been preceded yet. Direct microscopic identification of the organism in biopsies or inspired materials is the method of choice for diagnosing the organism. Advanced molecular techniques offer a high sensitivity and specificity in P. Jirovecii diagnosis. Sulfa-based medicines have been shown effectiveness in P. Jirovecii infections treatment. Aim of this review to provide an updated knowledge on the role of P. jirovecii as a human pathogen.