The fungus that causes histoplasmosis, courtesy CDC, via Wikipedia
Histoplasmosis is a fungal disease caused by the fungus histoplasma capsulatum. It is endemic to most of central North America and most people who live in areas where is common have been exposed to it. It is almost always asymptomatic and builds up antibodies against subsequent infection. It is common in areas contaminated with bird droppings or bat guano. It can present with several symptoms, but almost always involves the lungs. If it is limited to the lungs, it is rarely fatal, but it's mortality increases if it affects other organ systems. Individuals who are immunocompromised are particularly at risk for infections.
After exposure, symptoms usually appear within three weeks. They usually present as non-specific lung problems which can easily be mistaken for influenza. Chronic forms of the disease can be mistaken for tuberculosis. A lung x-ray of a patient with the disease will look normal in about half of cases. There is an antibody test, but in most cases it is a false positive and only shows the person was exposed in the past. However, it will show up in blood cultures, sputem cultures and cultures taken from infected organs.
Treatment is with anti-fungal drugs. However, if the disease is asymptomatic, treatment is contra-indicated as the side effects of the drugs are usually more dangerous than the danger of a relapse.