Pneumocystis jirovecii (formerly known as P. Carinii) is a yeast-like fungus of the genus Pneumocystis and is the agent that causes Pneumocystis Pneumonia (Known as PJP and previously known as PCP)
It most commonly affects the immunocompromised and, in some cases, can be severely life-threatening. Typically, patients at risk are those with any underlying disease states that alter host immunity such as those with cancer, the HIV, transplant recipients, or those taking immunosuppressive therapies and medications.