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.

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.

How does the infection develop?

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.

There are five different forms of infection, as follows:

• Asymptomatic is recognized only by performing medical laboratory tests as the victim does not show any symptoms and is unaware of the infection.
• Acute disseminated does not last long (i.e., acute) but it involves other organs outside the lungs (i.e., disseminated). It is usually confined to infants and young children and is marked by fever, cough, exhaustion and enlargement of the liver and spleen.
• Acute benign respiratory is produced by a heavy exposure to conidia. It is marked by weakness, fever, chest pains, and cough. The severity of the symptoms depends upon the magnitude of the exposure.
• Chronic disseminated is of long duration (chronic) and it involves other organs outside of the lungs. It occurs in people with a reduced capacity to fight disease, for example, in patients with leukemia (cancer of the system producing blood cells) and in persons being treated with drugs that suppress the body's defense mechanisms against diseases. The chronic disseminated form is marked by fever, anemia, hepatitis, pneumonia, inflammation of the lining of the heart cavity, meningitis, and ulcers of the mouth, tongue, nose and larynx.
• Chronic pulmonary occurs in persons with pre-existing lung diseases such as emphysema. It resembles tuberculosis and is more common in males over 40 years of age.