How did the international and domestic war effort cause the 1918 flu pandemic to spread more easily?

As soldiers in the trenches became sick, the military evacuated them from the front lines and replaced them with healthy men. This process continuously brought the virus into contact with new hosts—young, healthy soldiers in which it could adapt, reproduce, and become extremely virulent without danger of burning out.

it was difficult to control crowding in the training camps, it was impossible in the battlefields. Evolutionary biologist Paul Ewald has argued that trench warfare and its crowded conditions enabled an especially aggressive and deadly influenza virus to gain footing in humans.15 As soldiers in the trenches became sick, the military evacuated them from the front lines and replaced them with healthy men. This process continuously brought the virus into contact with new hosts—young, healthy soldiers in which it could adapt, reproduce, and become extremely virulent without danger of burning out. From there, according to a Navy report, “It is reasonable to suppose that late in August influenza of severe type was spread from French, Spanish, and Portuguese seaports to the Orient, South Africa, the United States, and South America.”5 (p. 2427) As Chesney and Ewald suggest, the influenza of 1918 was a product of trench warfare, and the influenza that attacked the 6th Artillery at Valdahon would travel the highways of war, circling the globe.