Background Histoplasmosis is caused by infection with Histoplasma capsulatum and was first described by pathologist Samuel Darling in Panama, who mistakenly thought it was a protozoan. It was not until the 1930s that the organism was correctly understood to be mold at room temperature and a yeast at 37 C after being isolated growing on artificial media. Because histoplasmosis causes pulmonary calcifications (particularly noted on chest radiograph) many of the early cases were mistaken for pulmonary tuberculosis. With the advent of a skin test reagent prepared from the yeast phase of the organism, the disease was found to be wide-spread in the Ohio and Misssissippi River Valleys in the United States.