
Spanish flu
The 1918–1920 flu pandemic was caused by H1N1. Earliest documented cases were in Haskell County, Kansas, in March 1918. It infected ~500 million people. Death estimates range from 17 million to 100 million. Wartime censorship created the false impression that Spain was the origin. Its unusually high mortality among young adults likely resulted from immune overreaction compounded by war-induced malnourishment and overcrowding. Geographic origin remains indeterminate.

