Medical Definition of Spanish flu
: influenza that is caused by a subtype (H1N1) of an orthomyxovirus (species Influenza A virus of the genus Influenzavirus A) and that was responsible for about 500,000 deaths in the U.S. in the influenza pandemic of 1918–1919 Between 20 million and 40 million died around the globe of the Spanish flu of 1918, probably the worst pandemic in human history.—Douglas M. Birch, Baltimore Sun, 16 Dec. 1997—compare asian flu, hong kong flu
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