Disease caused by Histoplasma capsulatum, a dimorphic fungus, typically found as mycelia/conidia (i.e. hyphae and spores) in soilof North and Central America, but also other parts of the Tropics. The typical route of entry into the human host is via inhalation whereupon it switches to the yeast form. This is accompanied by a shift in gene expression.
The yeast form is avidly taken up by macrophages, but various virulence factors impair the ability of the macrophages to destroy the fungus. The macrophages carry the organism into the reticuloendothelial system, disseminating the infection. The immune system eventually controls the infection, but the infection is never completely eradicated.
In endemic areas, Histoplasma exposure is extremely common. The initial infection in an immunocompetent host producing a mild respiratory illness or no symptoms at all, but more severe disease may occur in times of immunosuppresion.