The 1918 influenza pandemic was caused by an H1N1 virus of avian origin that spread worldwide in successive waves between 1918 and 1920. It infected roughly a third of the world's population, around 500 million people, and killed an estimated 50 million, with an unusually high toll among healthy young adults. Wartime troop movements and crowded conditions sped its global transmission, and cities responded with closures of schools, theatres and public gatherings, mask orders and quarantine. The lethal second wave in autumn 1918 caused the bulk of the deaths.
It was the deadliest pandemic of the 20th century and reshaped public-health practice, demonstrating how mass mobility and urban density accelerate disease. It strengthened arguments for sanitation, ventilation and open space that had long informed urban reform.
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