Histoplasmosis is a respiratory disease caused by a fungus that grows in soil enriched by animal droppings, including those from bats. Ninety percent of all reported cases in humans come from the Ohio and Mississippi River valleys and adjacent areas where warm, humid conditions favor fungal growth. The disease is rare or nonexistent in most of Canada and in the far northern and western United States. The majority of cases are asymptomatic or involve flu-like symptoms, though some individuals become seriously ill, especially if exposed to large quantities of spore-laden dust. To be safe, avoid breathing dust in areas where there are animal droppings; if you must clean an area of bat or bird droppings, wear a respirator that can guard against particles as small as two microns.