Pneumocystis carinii pneumonia (PCP) is an infection of the lung (pneumonia) caused by a parasite called Pneumocystis carinii. The parasite appears to be in the lungs of almost everyone, presumably acquired sometime early in life. {Pneumocystis carinii is found almost exclusively in the lungs, but like most-facts in medicine, this is not an absolute, and pneu-mocystis infections have occasionally occurred in other parts of the body.) The parasite causes no problem unless the balance between the immune system and the microbial world is heavily tilted in favor of the microbe. As a result, physicians initially saw PCP mostly in malnourished children, in people with certain types of cancer, and in those who had received organ transplants. At present, over 95 percent of people with PCP have HIV infection.     The reason so many people with HIV infection get PCP is probably that the CD4 cell, which is infected by HIV, is also important in keeping Pneumocystis carinii in check. As the supply of CD4 cells decreases, Pneumocystis carinii is less constrained. In approximately 50 percent of the people with HIV infection, PCP is the first AIDS-defining diagnosis. This means that this infection is the first condition that meets the CDC’s criteria for AIDS; it also means that the loss of immune defenses is relatively severe. Without treatment, about 8t) percent of all people with AIDS will develop PCP at some time during the course of the disease.     PCP in people with AIDS evolves slowly. In fact, most people have had the symptoms for weeks before they seek medical attention. The disease can be serious, and at first approximately 25 percent of people with PCP died. The prognosis is better now because of earlier medical intervention and improved treatments.
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