A person with histoplasmosis has an infection caused by a common fungus found in nature, called Histoplasma capsulatum. The infection starts in the lungs, but can spread to other areas of the body.

In the US, almost all people living in the Midwest are infected with histoplasmosis. Only about 5 out of 100 infected persons develop symptoms of infection. About 99% of patients with histoplasmosis improve without treatment.