US Army, 1918 influenza pandemic. US soldiers and patients at a cinema wearing masks during the 1918 influenza pandemic. The 1918 Spanish flu 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 including soldiers. Around half the US casualties in Europe were due to the pandemic. These soldiers and patients are from US Army Hospital No. 30, Royat, France.