Histoplasma capsulatum fungus, illustration. Histoplasma capsulatum is a species of parasitic, yeast-like dimorphic fungus that can, if inhaled, cause a type of lung infection 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 against the disease. H. capsulatum has a mycelial form (seen here) which is found in soil enriched by animal excrement. The spores (purple) are produced by the hyphae, the threadlike structures.