The pandemic that killed millions worldwide during the final year of World War I was the Spanish flu.
Also called the 1918 influenza pandemic, it was caused by an H1N1 strain of influenza A. Major outbreaks spread during 1918, with especially severe waves in the spring and autumn. Wartime troop movements, crowded military camps, hospitals, ships, and trenches helped the respiratory virus travel rapidly across continents.
The pandemic’s common name is misleading. Spain was neutral during World War I, so its newspapers could report the outbreak more openly than newspapers in combatant countries, where censorship protected military morale. Those reports made Spain appear to be the source, although the disease’s precise geographic origin remains uncertain.
An estimated 500 million people—roughly one-third of the world’s population at the time—were infected. Death estimates generally range from 25 million to 50 million, though historians and epidemiologists debate the total. Unlike typical seasonal influenza, the pandemic also caused unusually high mortality among some young adults. It continued in waves after the November 1918 armistice.