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Pneumocystis jirovecii (formerly known as Pneumocystis carinii) is a fungus found in the lungs of mammals where they reside without causing overt infection unless the host's immune system becomes compromised, at which time an often lethal pneumonia can develop. This fungus is found in humans and animals worldwide. Serologic evidence indicates that most healthy children have been exposed by age 3 to 4. Pneumocystis pneumonia (PCP) occurs in immunosuppressed individuals and in premature, malnourished infants.