Influenza virus

The family of Orthomyxoviridae contains 3 genera : Influenzavirus A, B; Influenzavirus C; and “Thogoto-like viruses.” Each type of virus has a stable nucleoprotein group antigen common to all strains of the type, but distinct from that of the other type; the genome is negative sense single-stranded RNA in 6–8 segments; each also has a mosaic of surface antigens (hemagglutinin and neuraminidase) that characterize the strains and that are subject to variations of two kinds: 1) a rather continual drift that occurs independently within the hemagglutinin and neuraminidase antigens; 2) after a period of years, a sudden shift (notably in type A virus of human origin) to a different hemagglutinin or neuraminidase antigen. The sudden major shifts are the basis of subdivisions of type A virus of human origin, which occur following infection of the animal host with 2 different strains at the same time, resulting in a hybrid virus. Strain notations indicate type, geographic origin, year of isolation, and, in the case of type A strains, the characterizing subtypes of hemagglutinin and neuraminidase antigens ( e.g., A/Hong Kong/1/68 (H₃ N₂); B/Hong Kong/5/72).

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influenza virus n any of the orthomyxoviruses that belong to three genera (Influenzavirus A, Influenzavirus B, and Influenzavirus C) and that cause influenza A, influenza B, and influenza C in vertebrates

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any of a group of orthomyxoviruses that cause influenza, including at least three genera: Influenzavirus A, Influenzavirus B, and Influenzavirus C. Antigenic variants are classified on the basis of their surface antigens (hemagglutinin and neuraminidase) as H1N1, H2N2, etc. Serotype A viruses are subject to major antigenic changes (antigenic shifts) as well as minor gradual antigenic changes (antigenic drift) and cause the major pandemics. Serotype B viruses appear to undergo only antigenic drift and cause more localized epidemics. Serotype C viruses appear to be antigenically stable and cause only sporadic disease. See Plate 49.