The 1918 flu pandemic (the "Spanish flu") was an influenza pandemic. It was an unusually deadly and severe 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.
The pandemic lasted from January 1918 to December 1920, spreading even to the Arctic and remote Pacific islands. Between 20 and 50 million died, making it one of the deadliest natural disasters in human history. Using the higher estimate of 50 million people, 3% of the world's population (which was 1.86 billion at the time) died of the disease. Some 500 million, or 27%, were infected.
Tissue samples from frozen victims were used to reproduce the virus for study. This research concluded, among other things, that the virus kills through a cytokine storm (overreaction of the body's immune system), which perhaps explains its unusually severe nature and the concentrated age profile of its victims. The strong immune system reactions of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults resulted in fewer deaths.