The Spanish Flu of 1918

The Spanish flu (Spanish: La Gripe Española), also known as the 1918 flu pandemic or La Pesadilla (Spanish for "The Nightmare"), was an unusually deadly influenza pandemic. Lasting from January 1918 to December 1920, it infected 500 million people – about a quarter of the world's population at the time. The death toll is estimated to have been anywhere from 17 million to 50 million, and possibly as high as 100 million, making it one of the deadliest pandemics in human history, behind the Black Death.

Source: Wikipedia.