Dysentery causes inflammation of the intestines, especially the colon, and results in bleeding during defecation and diarrhea. The disease is spread through a lack of sanitation and malnutrition, especially in filthy places like trenches and prison camps. It is usually fatal if left untreated, and resulted in thousands of deaths during WWI.
World War I was not the only time the disease wreaked havoc on an army. The Grande Armée of Napoleon Bonaparte lost 80,000 men to dysentery during the invasion of Russia.