Spanish flu ambulances. Red Cross Motor Corps ambulance workers on duty in October 1918 in St Louis, Missouri, USA. The women are wearing masks to reduce the spread of infection. 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 including soldiers.