Species of the genus influenzavirus b that cause HUMAN INFLUENZA and other diseases primarily in humans. antigenic variation is less extensive than in type A viruses (influenza a virus) and consequently there is no basis for distinct subtypes or variants. epidemics are less likely than with influenza a virus and there have been no pandemics. Previously only found in humans, influenza b virus has been isolated from seals which may constitute the animal reservoir from which humans are exposed.