Influenza ward. Part of a US army camp hospital in Langres, France. This ward dealt with soldiers infected with influenza, or flu. After the First World War there was a global influenza pandemic, known as the Spanish Flu. 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.