What disease killed the most soldiers in WW1?

The Silent Killer of the Great War

While trench warfare and artillery bombardment define our collective memory of World War I, an invisible enemy proved just as devastating on the battlefield. The 1918 influenza pandemic, commonly known as the Spanish Flu, swept through military camps worldwide, rapidly becoming the single largest disease killer of the war.

For the American Expeditionary Forces (AEF), the biological crisis was catastrophic. Official records documented over 192,000 cases of influenza and 29,000 cases of pneumonia, resulting in 13,000 deaths within the AEF alone. War Department estimates revealed that roughly 25% of the U.S. Army—more than one million men—fell ill during the outbreak. The cramped quarters of troop ships, crowded barracks, and damp trenches provided ideal conditions for the virus to spread unchecked.

Across the entire United States military, influenza and its lethal complication, pneumonia, claimed nearly 30,000 lives. This fatal combination accounted for more than half of the 52,000 non-combat deaths suffered by American forces during the conflict. Ultimately, global troop movements accelerated the spread of the virus, proving that unseen pathogens could be as destructive as any weapon on the Western Front.