In Modern Germs, Alex Tabarrok cites a number of stats from John M. Barry’s The Great Influenza:
The great influenza of 1918 probably killed 100 million people, about five percent of the entire world’s population. An even higher percentage of young people died, and most shockingly all of this occured in about 12 weeks.
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One doctor visiting Inuit in Alaska found everyone dead in 3 villages and 7 other villages with a death toll of 85%.