The Spanish Flu or the 1918 flu pandemic was an 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.

The pandemic lasted from March 1918 to June 1920, spreading even to the Arctic and remote Pacific islands. Between 50 and 100 million died, making it one of the deadliest natural disasters in human history. An estimated 50 million people, about 3% of the world's population (1.6 billion at the time), died of the disease. Some 500 million, or 1/3 were infected.

Tissue samples from frozen victims were used to reproduce the virus for study. Among the conclusions of this research is that the virus kills via 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 systems of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults resulted in fewer deaths.