Influenza-B virus is a genus in the virusfamily Orthomyxoviridae. The only species in this genus is called "Influenza B virus". Influenza B virus only infects humansand seals. This limited host range is apparently in contrast with those caused by the similar Influenza virus A as both mutate by both genetic drift and reassortment. Influenza-B virus evolves slower than A viruses and faster than C viruses. Influenza-B virus mutates at a rate 2-3 times lower than type A. However， influenza B mutates enough that lasting immunity is not possible. The Influenza B virus capsidis enveloped while its virionconsists of a matrix protein + envelope + nucleoprotein complex + nucleocapsid， and a polymerasecomplex. Influenza B is sometimes spherical and sometimes filamentous. Its 500 or so surface projections are made of hemagglutinin and neuraminidase.
The Influenza B virus is 14648 nucleotideslong and consists of eight segments of linear negative-sense， single-stranded RNA. The multipartite genome is encapsidated， each segment in a separate nucleocapsid， and the nucleocapsids are surrounded by one envelope.
