This paper examines medical research and sanitary orders of the United States in Europe during World War I. It aims to analyze the relationships between public health and war in terms of epidemics and military interests of the United States. To cope with Serbian typhus fever in 1915, the United States, which had not been directly involved in war, delegated a sanitary commission organized by private agencies. As the characters of typhus fever had been well known at that time, the sanitary commission aimed at curing patients and hindering additional cases by improving hygienic conditions. After the United States determined to enter the war and dispatched troops in 1917, however, the approach to epidemics in Europe changed. To handle the yet unknown trench fever that had undermined Allied military power, the U.S. Army organized the medical corps for scientific investigations. Medical corps conducted experiments on voluntary soldiers, and concluded that trench fever was caused by the bite of louse and could be transmitted through blood. Medical research and sanitary orders to two epidemics during the same war had developed differently according to the characters of each disease and military interests of the United States.