Illustration of spores of the yeast-like fungus Histoplasma capsulatum. When inhaled these spores can cause a respiratory disease called histoplasmosis. The primary pulmonary form is usually harmless. Occasionally a progressive histoplasmosis develops which resembles tuberculosis. Histoplasmin, an antigenic material produced by the fungus, is used to test for the disease. H. capsulatum has a mycelial form which is found in soil enriched by animal excrement.