The 1918 influenza pandemic (January 1918 – December 1920; colloquially known as Spanish flu) was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus, with the second being the swine flu in 2009. It infected 500 million people around the world, or about 27% of the then world population of between 1.8 and 1.9 billion, including people on isolated Pacific islands and in the Arctic.
The death toll is estimated to have been 40 million to 50 million, and possibly as high as 100 million, making it one of the deadliest epidemics in human history.
And guess where it came from?