FROM WIKIPEDIA q.v.
The 1918 flu pandemic (commonly referred to as the Spanish flu) was an influenza pandemic that spread to nearly every part of the world. It was caused by an unusually severe and deadly Influenza A virus strain of subtype H1N1. Historical and epidemiologic data are inadequate to identify the geographic origin of the virus.[1]
Many of its victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients. The Spanish flu lasted from March 1918 to June 1920,[2] spreading even to the Arctic and remote Pacific islands. It is estimated that anywhere from 20 to 100 million people were killed worldwide, or the approximate equivalent of one third of the population of Europe,[4][5][6] more than double the number killed in World War I.[7]
This extraordinary toll resulted from the extremely high illness rate of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms.