Histoplasma: Full name: histoplasma capsulatum. A microscopic fungus that causes the disease histoplasmosis. The fungus is found throughout the world in river valleys and soil where bird or bat droppings accumulate. The spores of the fungus are released into the air when contaminated soil is disturbed (for example, by plowing fields, sweeping chicken coops, or digging holes) and the airborne spores can then be inhaled into the lungs, the primary site of infection. In the US, the fungus is so common that in parts of Kentucky and Tennessee nearly 90% of adults show evidence of exposure by a positive histoplasma skin test. The fungus can be transferred by organ transplantation.