Excerpts from Books and Wikipedia
"The Spanish flu, also known as the 1918 flu pandemic, was an unusually deadly influenza pandemic caused by the H1N1 influenza A virus. Lasting from February 1918 to April 1920, it infected 500 million people – about a third of the world's population at the time – in four successive waves. The death toll is typically estimated to have been somewhere between 20 million and 50 million, although estimates range from a conservative 17 million to a possible high of 100 million, making it one of the deadliest pandemics in human history." [Wikipedia]