The 1918 Influenza Pandemic
In the last year of the war, an influenza strain circled the globe on troop ships and trains and killed somewhere between 50 and 100 million people — more than the war itself, and unusually it killed the young and healthy rather than the frail. Wartime censors suppressed reports to protect morale; neutral Spain reported freely, so the world blamed Spain and the name stuck. It was demographic catastrophe on the scale of the Black Death, and it was largely forgotten within a decade.
A pathogen spreads through a population and reshapes its demography, economy, and beliefs at once.