What was the 1918 Spanish flu, and why was it called that?

The 1918 influenza pandemic infected roughly a third of the world's population and killed an estimated 50 million people, with some estimates higher, in three waves between spring 1918 and 1919. Unusually, it killed young healthy adults at high rates rather than mainly the very young and old, a pattern attributed to an overwhelming immune response. In the United States about 675,000 died, and life expectancy dropped by around twelve years in a single year. It spread through military camps and troop ships in the final months of the First World War, which is why the war and the disease are entangled in the history.

The name is a historical accident. Wartime censorship in Germany, France, Britain, and the United States suppressed reporting on the outbreak to protect morale. Spain was neutral and its newspapers reported freely, including on the illness of King Alfonso XIII, so the disease appeared to be centered there and the name stuck. Spain itself called it the French flu. The actual geographic origin is debated, with candidates including a Kansas army camp, northern France, and China, and it is not resolved.

The pandemic shaped public health in lasting ways. Cities that imposed early and sustained closures of schools, churches, and gatherings, such as St. Louis, had markedly lower death rates than those that delayed, such as Philadelphia, which held a large parade during the outbreak, and that comparison became a foundational case study for the concept of flattening the curve. The virus was reconstructed from preserved tissue samples in 2005 and identified as an H1N1 strain, and its descendants circulated in seasonal flu for decades. It was also substantially forgotten in popular memory for most of the twentieth century, a gap historians attribute to its overlap with the war and to the absence of any institution built to remember it.