Histoplasma capsulatum is a species of dimorphic fungus. Its sexual form is called Ajellomyces capsulatus. It can cause pulmonary and disseminated histoplasmosis. H. capsulatum is "distributed worldwide, except in Antarctica, but most often associated with river valleys" and occurs chiefly in the "Central and Eastern United States" followed by "Central and South America, and other areas of the world". It is most prevalent in the Ohio and Mississippi River valleys. It was discovered by Samuel Taylor Darling in 1906.