Diarrhea, a scourge upon humanity since preliterate times, has been the particular nemesis of military forces. The Armed Forces of the United States have been in the forefront in the diagnosis, treatment, and prevention of diarrheal illness. U.S. military scientists and physicians implemented the first mandatory typhoid inoculation program, contributed to advances in water chlorination, and pioneered the use of antibiotics for typhoid fever. U.S. Navy physicians refined the intravenous treatment of cholera, reducing the death rate from 20% to less than 1%. Their studies of electrolyte and fluid balance in cholera, and the subsequent development of oral rehydration therapy for cholera and other diarrheal illness, have saved millions of lives worldwide. U.S. Army researchers refuted the desquamation theory of cholera pathogenesis, isolated the cholera exotoxin, and developed improved cholera vaccines. U.S. Army and Navy researchers pioneered the use of antibiotics for the treatment of typhoid fever, made major contributions to the treatment of dysentery, developed algorithms for the treatment of traveler’s diarrhea, and continue active development of traveler’s diarrhea and dysentery vaccines. U.S. military diarrheal research has directly contributed to the welfare of hundreds of millions of people.