Men queuing to receive a Spanish flu serum injection during the Spanish Flu pandemic. A number of vaccines were trialled during the pandemic, however the H1N1 virus that caused the outbreak had not been identified and so none were effective. The 1918 Spanish influenza pandemic, which occurred in several waves between 1918 and 1920, infected one fifth of the world population and killed between 20 and 50 million, more than had been killed in the First World War. The second wave of the pandemic, from August 1918, was much deadlier than the first, with high mortality rates among young healthy adults. Photographed in Seattle, Washington, USA, in 1918.