Protection against influenza, 1918. US soldiers wearing gauze masks to protect against influenza (flu). The masks were totally ineffectual. 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. Photographed in Seattle, USA.