Histoplasma capsulatum is a fungus that primarily grows in soil enriched with bird or bat droppings. It is commonly found in certain regions of the United States, particularly in the Ohio and Mississippi River valleys. When the spores of this fungus are inhaled, they can cause a respiratory infection known as histoplasmosis. Histoplasmosis can range from mild to severe, depending on the individual's immune system. Most healthy people recover without treatment, but those with weakened immune systems may experience more serious symptoms. Diagnosis typically involves laboratory tests, and antifungal medications may be prescribed for severe cases.