MEDLINEPLUS: <p><em>Pneumocystis jirovec</em> is a tiny fungus that lives in the lungs of many people. Most people's immune systems keep the fungus under control. But if your immune system is weak, the fungus can make you very sick. </p><p>The most common problem of infection is pneumocystis pneumonia (PCP). PCP once was the major cause of death for people with <a href='http://www.nlm.nih.gov/medlineplus/hivaidsandinfections.html'>HIV/AIDS</a>. But now, it is possible to prevent or treat most cases. The key to surviving PCP is early treatment. The first signs of PCP are difficulty breathing, fever and a dry cough. If you have these symptoms, see your doctor right away.</p>,MSH: Infections with species in the genus PNEUMOCYSTIS, a fungus causing interstitial plasma cell pneumonia (PNEUMONIA, PNEUMOCYSTIS) and other infections in humans and other MAMMALS. Immunocompromised patients, especially those with AIDS, are particularly susceptible to these infections. Extrapulmonary sites are rare but seen occasionally.