Illustration of the spore of a yeastlike fungus called 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, is produced from the fungus to test against the disease. H.capsulatum has a mycelial form which is found in soil enriched by animal excrement.