Spanish Flu (1918-1919)

The influenza pandemic of 1918-1919 infected approximately 500 million people — one third of the world's population — and killed 50 to 100 million, including disproportionate numbers of young adults in the 20 to 40 age group unlike ordinary flu seasons. It killed more people in a year than the four years of World War I, collapsing healthcare systems worldwide and influencing the 1919 Paris Peace Conference negotiations as key leaders fell ill.