Approximately how many people have had histoplasmosis or been exposed to it?

• A. 50 million
• B. 1 million
• C. 10 thousand
• D. 500 thousand

Answer: (A)

Histoplasmosis exposure is extremely common because Histoplasma capsulatum thrives in soil enriched with bird or bat droppings, especially in areas like the Ohio and Mississippi River valleys. A large portion of people living in or visiting these areas has inhaled spores at some point, and many exposures never cause noticeable illness. Serologic and epidemiologic data show widespread past exposure, so the total number of people who have had histoplasmosis or been exposed accumulates to tens of millions. In this context, about 50 million is a reasonable estimate. The smaller numbers don’t reflect how broad and long-standing exposure is across the population.