Influenzavirus B is a genus in the virus family Orthomyxoviridae. Influenza B viruses infects only humans and seals. Influenzavirus B mutates at a rate 2 to 3 times lower than type A. Type B flu may cause a less severe reaction than type A flu virus, but occasionally, type B flu can still be extremely harmful. Influenza type B viruses are not classified by subtype and do not cause pandemics.