The American Civil War represents a landmark in military and medical history as the last large-scale conflict fought without knowledge of the germ theory of disease. Unsound hygiene, dietary deficiencies, and battle wounds set the stage for epidemic infection, while inadequate information about disease causation greatly hampered disease prevention, diagnosis, and treatment. Pneumonia, typhoid, diarrhea/dysentery, and malaria were the predominant illnesses. Altogether, two-thirds of the approximately 660,000 deaths of soldiers were caused by uncontrolled infectious diseases, and epidemics played a major role in halting several major campaigns. These delays, coming at a crucial point early in the war, prolonged the fighting by as much as 2 years.