Transmitting the Flu
The trenches, where much of the fighting took place during the Great War, were deadly in many ways. In trench warfare, soldiers protected themselves from enemy fire by fighting from below ground in deep but narrow dugouts surrounded by barbed wire. Conditions in these trenches were atrocious by modern standards. Damp, cold, dirty, vermin-infested, and sometimes covered with human remains, they were the perfect setting for viral mutation, hastening the spread of the disease through close contact. These suboptimal conditions were not limited to close spaces like battlefield trenches or crowded barracks. Once contracted, influenza travelled with victims as they moved from place to place, whether on foot, by train, or aboard ships. Realizing this, American medical officers attempted to convince the War Department to reduce crowding on ships and other transport, but to no avail. Consequently, all forms of troop and supply transport continued to spread the virus globally.
While it's tempting to think that stronger efforts to contain influenza where it raged would have stopped its spread, this would have been impractical during wartime, when the advantage gained by moving troops and equipment quickly and efficiently was crucial. Such efforts might have slowed the spread of influenza, but it is unlikely they would have completely stopped transmission because of the impossibility of total quarantine.