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. It infected 500 million people around the world, including people on remote Pacific islands and in the Arctic. The death toll is estimated to have been 50 million, and possibly as high as 100 million (three to five percent of Earth's population at the time), making it one of the deadliest epidemics in human history.
Academic Andrew Price-Smith has made the argument that the virus helped tip the balance of power in the latter days of WWI towards the Allies. He provides data that the viral waves hit the Central Powers before the Allied powers and that both morbidity and mortality in Germany and Austria were considerably higher than in Britain and France.