Histoplasmosis, hihs tuh plaz MOH sihs, is a disease that attacks the lungs, mouth, throat, ears, bone marrow, spleen, liver, and lymphatic system. It is caused by a fungus called Histoplasma capsulatum. The infection begins in the lungs and is usually not dangerous. Most people suffer only mild cases with flulike symptoms or no symptoms at all. But infants and patients with AIDS may develop severe cases. Histoplasmosis can be fatal if it spreads to other parts of the body. It is not contagious.