Histoplasma capsulatum [Gk, histos + plasma + L, capsula, little box] , a dimorphic fungal organism that is a single budding yeast at body temperature and a mold at room temperature. It is the causative organism in histoplasmosis, common in the Mississippi River Valley. The fungus, spread by airborne spores from soil contaminated with excreta from birds or infected bats, acts as a parasite on the cells of the reticuloendothelial system. See also histoplasmosis.
