Medical Definition of influenza
1a: an acute, highly contagious, respiratory disease caused by any of three orthomyxoviruses: (1) or influenza A : moderate to severe influenza that in humans is marked especially by sudden onset, fever, sore throat, fatigue, muscle aches, inflammation of the respiratory mucous membranes, and cough, is caused by any of several subtypes (such as H1N1, H2N2, or H3N2) of an orthomyxovirus (species Influenza A virus of the genus Influenzavirus A) infecting humans and various animals (such as birds or pigs), and may occur in seasonal epidemics or sometimes pandemics following mutation of the causative virus—see asian flu, avian influenza, bird flu, hong kong flu, spanish flu, swine flu, swine influenza (2) or influenza B : influenza with less severe symptoms that is caused by an orthomyxovirus (species Influenza B virus of the genus Influenzavirus B) infecting only humans and especially children and that may occur in seasonal epidemics but not pandemics (3) or influenza C : mild influenza that is often asymptomatic, is caused by an orthomyxovirus (species Influenza C virus of the genus Influenzavirus C) infecting only humans, and does not occur in epidemics or pandemicsb: any human respiratory infection of undetermined cause—not used technically
2: any of various viral diseases of domestic animals (as shipping fever of horses) marked especially by fever, respiratory symptoms, and inflammation of mucous membranes ◆All types of influenza are commonly called “the flu.”
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