Spanish Flu

The 1918 flu pandemic (the Spanish Flu) was an unusually severe and deadly influenza pandemic that spread across the world. Historical and epidemiological data are inadequate to identify the geographic origin. Most victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or weakened patients. The flu pandemic was implicated in the outbreak of encephalitis lethargica in the 1920s. (source: Wikipedia)