influenza Definition

an acute, highly contagious, respiratory disease caused by any of three orthomyxoviruses:

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, that has numerous variants caused by 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 that may occur in seasonal epidemics or sometimes pandemics following mutation in the causative virus

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

highly variable influenza A of birds caused by any of the subtypes of the causative orthomyxovirus of which some strains have been or may be transmitted to other vertebrates including humans especially after undergoing mutation —called also avian flu

influenza A of swine that is caused by any of several subtypes (such as H1N1 or H3N2) of the causative orthomyxovirus and in which sporadic transmission to humans has occurred

influenza that is caused by a subtype (H2N2) of the orthomyxovirus causing influenza A and that was responsible for about 70,000 deaths in the U.S. in the influenza pandemic of 1957—58 —called also Asian influenza