Of the 620,000 soldiers who perished during the American Civil War, the overwhelming majority died not from gunshot wounds or saber cuts, but from disease. In this ground-breaking medical history, Andrew McIlwaine Bell explores the impact of two terrifying mosquito-borne maladies--malaria and yellow fever--on the major political and military events of the 1860s, revealing how deadly microorganisms carried by a tiny insect helped shape the course of the Civil War.