Histoplasmosis is an infection caused by Histoplasma capsulatum, a soil fungus. Exposure occurs by inhalation, and the primary infection is in the lung. The disease is usually benign and self-limited, despite a strong tendency for invasion of the bloodstream during the primary infection. This fungemia seeds reticuloendothelial organs throughout the body. Under favorable conditions, the organism can cause progressive disease in one or in multiple sites, resulting in a wide variety of clinical manifestations.

Distribution and Incidence

H. capsulatum has been isolated from soil of more than 50 countries. It is most common in temperate climates along river valleys and has been found in North, Central, and South America; India; Southeast Asia; and rarely Europe.

By far the most heavily endemic area in the world is the east central United States, particularly the Mississippi and Ohio River valleys. It is most prevalent in the states of Ohio, Kentucky, Indiana, Illinois, Kentucky, Tennessee, and Arkansas. Surrounding states also have many infections.

Infection is almost universal in the most heavily endemic areas. Skin test surveys reveal that over 90 percent of persons living in some counties in the central United States have had histoplasmosis before age 20 (Edwards et al. 1969). Based on skin test surveys, there are probably 40 to 50 million people in the central United States who have had histoplasmosis, and there are several hundred thousand new cases each year. The number of serious infections requiring diagnosis and treatment is very small, perhaps 1 or 2 percent of the total.