Histoplasmosis is a widespread fungal infection which occurs worldwide. In the US, it is most common in the southeastern, mid-Atlantic, and central states. The lungs are the portal of entry for this infection.

Histoplasma grows as a mold in soil, and infection results from inhalation of airborne fungal particles. Soil contaminated with bird or bat droppings may have a higher concentration of mold.

The skin lesions of histoplasmosis are varied and can be caused by an immune reaction to acute pulmonary (lung) involvement. In this case, lesions called erythema nodosum or erythema multiforme (target lesions) occur.

Skin lesions can also be a manifestation of disseminated histoplasmosis, where the fungus has spread to distant organs throughout the body including the skin, the bone marrow and the brain amongst others. Disseminated histoplasmosis is most common amongst immunosuppressed people, such as those with AIDS.