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Pneumocystis carinii is an organism that appears as minute oval bodies or cysts 5-10 mu metres in length. It is now believed to be a fungus, although it may yet be classified as a protozoan. It is a common commensal in the human lung, but in infants of a few months of age, or adults who are immunosuppressed, it can cause a debilitating pneumonia.
In Europe and North America, Pneumocystis carinii is the most common cause of pneumonia in patients who are antibody positive for HIV. It occurs when the CD4 positive count falls below 200 and up to 80% of all AIDS patients will suffer from P. carinii pneumonia at some stage. In Africa however P. carinii pneumonia is relatively unusual with tuberculosis being more common.
Pneumocystis carinii pneumonia is an illness that satisfies the diagnostic criteria for AIDS.