Histoplasmosis is an infection in the lungs caused by inhaling the spores of a fungus. This fungus, called Histoplasma capsulatum, is common in most of the central and eastern United States. The fungus grows in soil, as well as bird and bat droppings. It is spread by breathing in the spores of disturbed soil.

Many histoplasmosis infections do not produce symptoms. The illness occurs in two forms. The acute form is much like a mild case of influenza and is rarely serious.

The chronic form, which is much less common, may resemble tuberculosis. In rare cases, the disease will spread throughout the body. This is most likely to happen in the very young, the very elderly and people with impaired immune systems, including those with cancer or AIDS. If histoplasmosis has spread, it can be deadly if it is not treated.