Influenza poster, 1918. Poster displayed in American theatres during 1918, providing information about the influenza pandemic. After the First World War, there was a global influenza pandemic, known as the Spanish Flu (1918-1919). A fifth of the world's population was infected, and between 20 and 50 million people died. Such pandemics occur when a new infectious virus appears for which the human population has no immunity. This can happen when there is a major change in the surface proteins of an existing virus. This poster is from Chicago, USA.