Histoplasmosis is an infectious disease of the lungs caused by a fungus called Histoplasma capsulatum. The infection sometimes can spread to other parts of the body.

This Histoplasma organism thrives in moderate temperatures and moist environments. Droppings from chickens, pigeons, starlings, blackbirds, and bats support its growth. Birds are not infected with it because of their high body temperatures, but they do carry it on their feathers. Bats can be infected because they have a lower body temperature than birds and can excrete the organism in their droppings. Because bats populate them, caves often are sites where multiple people become infected simultaneously.

To multiply, Histoplasma capsulatum produces small spores called conidia. The conidia of Histoplasma capsulatum are only two millionths of a meter (microns, µm) in diameter. When these conidia are inhaled, they are small enough that they enter the lungs and start an infection. Many of these infections are easily overlooked because they either produce mild symptoms or none at all. However, histoplasmosis can be severe and produce an illness similar to tuberculosis.

When a person breathes in the conidia (spores) of Histoplasma capsulatum, the lungs' defense mechanisms attempt to neutralize them. Not all the conidia are neutralized. The ones that avoid the defense start an infection. The symptoms of the infection appear within 5 to 18 days after exposure, most commonly in 10 days.

Confirmation of a diagnosis of histoplasmosis often requires laboratory examinations which identify histoplasmosis in lung biopsy. Blood and urine antigen tests, skin antibody tests, and x-rays may be performed but they are only of secondary value in diagnosis. CT scans are often valuable in spotting damage done by one's immune system in its attempt to fight histoplasmosis.

Most patients who develop histoplasmosis do not require treatment. Some may only require supportive treatment that relieves the symptoms of the disease. Severe symptoms with a large involvement of the lungs, other organs, and/or bone marrow require treatment with specific antifungal drugs. It is highly recommended that people with both an immune disorder and histoplasmosis receive prompt antifungal treatment. Patients with compromised immune systems should always receive antifungal therapy; untreated disseminated histoplasmosis is usually fatal.

Histoplasmosis article courtesy of the National Institute of Environmental Health Sciences, National Institutes of Health, U.S. Department of Health and Human Services.

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