Histoplasmosis: A disease caused by the fungus Histoplasma capsulatum. Most people with histoplasmosis have no symptoms. However, it can cause acute or chronic lung disease and progressive disseminated histoplasmosis, which affects a number of organs. Infants, young children, and older persons'particularly those with chronic lung disease'are at increased risk for severe disease. Disseminated histoplasmosis is most frequently seen in people with cancer or AIDS. The acute respiratory disease of histoplasmosis is characterized by respiratory symptoms, a general ill feeling, fever, chest pains, and a dry or nonproductive cough. Distinct patterns may be seen on a chest x-ray. Chronic lung disease related to histoplasmosis resembles tuberculosis and can worsen over months or years. The disseminated form is fatal unless treated. Mild cases resolve without treatment. Severe cases of acute histoplasmosis and all cases of chronic and disseminated histoplasmosis are treated with antifungal medications, usually for life in those with compromised immune systems.