Pneumocystis jirovecii (previously P. carinii) is a yeast-like fungus of the genus Pneumocystis.

Pneumocystis pneumonia is an important disease of immunocompromised humans, particularly patients with HIV, but also patients with an immune system that is severely suppressed for other reasons, for example, following a bone marrow transplant. In humans with a normal immune system, it is an extremely common silent infection.