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former genus name for the viruses that cause influenza, now found to be two different genera, which were named influenzavirus A and influenzavirus B.
The family of Orthomyxoviridae contains three 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 types; 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 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 two 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).