The pandemic's first wave was documented in March 1918 at Camp Funston, a U.S. military base in Kansas; that same spring it crossed the Atlantic with American troops and reached the European front. The disease became known as the 'Spanish flu,' but the name is misleading: Spain was neutral in the First World War and, unlike the belligerent powers, did not censor its press — so the first open reports came from there, and the name stuck. The geographic origin of the virus remains debated; Kansas, northern China, and the Étaples military camp in France are among the candidates.

It spread in three waves: a mild first wave in March-July 1918, an extraordinarily lethal second wave in September-December 1918, and a third wave in early 1919. The triggering conditions had accumulated: wartime mobility moved millions of soldiers between continents, trenches and military camps were crowded and poorly ventilated, civilian health systems had been weakened by war. The H1N1 strain had an unusual feature — it disproportionately killed young adults aged 20-40, the group with the strongest immune systems; the likely cause was an over-reactive immune response now called a 'cytokine storm.'

Global death-toll estimates have been revised upward over time. Early 1920s counts gave 17-21 million; Patterson and Pyle's 1991 revision proposed 30 million; Johnson and Mueller's 2002 study estimated at least 50 million, with an upper bound of 100 million. India alone lost roughly 12-17 million people. For comparison: the First World War killed about 17 million over four years; the flu killed at least three times that in two. The end of the war in November 1918 coincided with the peak of the second wave — the peace celebrations were also mass-transmission events.

The pandemic's long shadow lasted for years. Public health infrastructure was rebuilt in many countries; national health ministries were established, and virology emerged as an independent science. Yet in cultural memory the Spanish flu remained in the shadow of the First World War and was long called 'the forgotten pandemic.' Until COVID-19 in 2020, it remained the deadliest single event of the 20th century and is the core reference point for modern epidemiology.