Medical Definition of H1N1
1: a virus that is a subtype (H1N1) of the orthomyxovirus (species Influenza A virus of the genus Influenzavirus A) causing influenza A, that infects birds, pigs, and humans, and that includes strains which may occur in seasonal epidemics or sometimes pandemics … by the third week of April it was established that the illness resulted from a triple recombination of human, avian, and swine influenza viruses; the virus has been found to be H1N1.—Lindsey R. Baden et al., The New England Journal of Medicine, 18 June 2009 Last year, older people had some immunity to the H1N1 virus because it resembled strains that circulated decades ago.—Blythe Bernhard, The St. Louis Post-Dispatch, 16 Jan. 2011 ◆The subtype H1N1 is distinguished by a mutation of hemagglutinin (H1) that affects the ability of the virus to infect cells, and a mutation of a neuraminidase (N1) that enables the release of the replicated virus from cells.
2: influenza caused by the H1N1 virus; especially : swine flu If you or your child came down with influenza during the H1N1, or swine flu, outbreak in 2009, it may not have happened the way you thought it did.—Nicholas Bakalar, The New York Times, 3 Feb. 2011 Thankfully, the H1N1 flu didn't turn into the doomsday bug that some scientists feared.—The Chicago Tribune, 17 Apr. 2010
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