Medical Definition of influenza A
: a common moderate to severe respiratory disease that affects humans and some other vertebrates (as swine and birds) sometimes in pandemics following mutation in the causative virus, that in humans is characterized by sudden onset, fever, prostration, severe aches and pains, and progressive inflammation of the respiratory mucous membranes, that is held to have aquatic birds as its natural and historic host, and that has numerous variants caused by subtypes (as H1N1, H2N2, or H3N2) of an orthomyxovirus (species Influenza A virus of the genus Influenzavirus A) distinguished especially by mutations of a hemagglutinin (as H1, H2, or H3) affecting the ability of the virus to infect cells and of a neuraminidase (as N1 or N2) involved in release of the replicated virus from cells—see avian influenza, swine influenza, ; asian flu, hong kong flu, spanish flu
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