Acute pulmonary histoplasmosis is a respiratory infection caused by inhaling the spores of the fungus Histoplasma capsulatum.

Histoplasma capsulatum, the fungus that causes histoplasmosis, is found in the Central and Eastern United States, Eastern Canada, Mexico, Central America, South America, Africa, and Southeast Asia. It is is commonly found in the soil along river valleys. It gets into the soil mostly from bird and bat droppings.

You can get sick when you breathe in spores produced by the fungus. Every year, thousands of people worldwide are infected, but do not become seriously sick. Most patients have no symptoms or have only a mild flu-like illness and recover.

Acute pulmonary histoplasmosis may happen as an epidemic, with many people in a particular geographical area becoming sick at the same time. Ongoing disease that continues to get worse can happen in people with impaired immune systems, such as those with HIV.

Risk factors include traveling to or living in the Central or Eastern United States near the Ohio and Mississippi River Valleys, and being exposed to the droppings of birds and bats. This threat is greatest after an old building is torn down, or when exploring caves. Having a weakened immune system increases your risk for getting the disease, and for having more and worse symptoms.