The Order Pneumocystidales is a group of fungi within the class Pneumocystidomycetes, classified under the phylum Ascomycota. This order comprises a single family, Pneumocystidaceae, which contains the genus Pneumocystis. These fungi are obligate parasites that inhabit the lungs of various vertebrates, including humans, where they can cause Pneumocystis pneumonia (PCP), particularly in immunocompromised individuals. The most studied species is Pneumocystis jirovecii, previously known as P. carinii, which is the causative agent of PCP in humans. Pneumocystis species are unique among fungi due to their inability to be cultured in vitro, making diagnosis reliant on molecular and histological techniques. They are transmitted via airborne routes, and infection often leads to severe respiratory illness if not treated promptly. The taxonomy of Pneumocystis has evolved over time, with genetic analyses confirming its classification as a fungus rather than a protozoan.