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 Aas 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.
