A lung infection caused by the fungus Histoplasma capsulatum. Spores of the fungus are found in bird and bat droppings and usually cause the disease by being inhaled. The disease can be mild, as in primary pulmonary histoplasmosis which is a type of pneumonia and resembles a cold or flu, or severe, as in progressive histoplasmosis where infection spreads from the lungs to other parts of the body, notably the liver and spleen, and causes skin lesions (most often as a mouth ulcer). The severe forms of the disease most commonly occur in people with immune system disorders such as AIDS, and are often fatal.