Medical Definition of influenza
1a: any of several acute highly contagious respiratory diseases caused by strains of three major orthomyxoviruses now considered to comprise three species assigned to three separate genera:: influenza a: influenza b: influenza cb: any human respiratory infection of undetermined cause—not used technically
2: any of numerous febrile usually virus diseases of domestic animals (as shipping fever of horses) marked by respiratory symptoms, inflammation of mucous membranes, and often systemic involvement
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