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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