At first glance, the Influenza B virus seems to be an easy virus to understand. Outdated textbooks and uninformed websites will still tell you that the virus only infects humans. That would suggest that we humans are the only known natural reservoir for the virus and that would also explain that there are only two subtypes (or strains) that circulate around the globe: Influenza B/Victoria and Influenza B/Yamagata.
But, just like all its family members, Influenza B evolves via antigenetic drift, antigenetic shift and reassortment. In humans, Influenza B viruses evolve slower than Influenza A viruses but faster than Influenza C viruses. The Influenza B virus evolves quickly enough to ensure that lasting immunity is not possible.
 indicated that the Influenza B virus was circulating in seals. Influenza B virus was isolated from naturally infected harbor seals (Phoca vitulina) and grey seals (Halichoerus grypus) that were living in the Wadden Sea, situated in the north of The Netherlands. Analysis indicated that influenza virus B/Seal/Netherlands/1/99 is closely related to strains that circulated in humans 4 to 5 years earlier. About 2% of seals after 1995 showed antibodies to the Influenza B virus but none from before 1995. The authors speculate that unvaccinated staff (!) from the Seal Rehabilitation and Research Centre in Pieterburen (The Netherlands) might be the cause of the first transmission.
Next, other seals around the globe were tested and - as far away as South America and the Caspian Sea - they all confirmed that Influenza B viruses were circulating in seals.
This possibly novel animal reservoir, harbouring Influenza B viruses may pose a direct threat to humans because now there’s a new mixing vessel in which the virus might evolve more quickly than ever before.
 Yamashita et al: Influenza B virus evolution: co-circulating lineages and comparison of evolutionary pattern with those of influenza A and C viruses in Virology - 1988
 Osterhaus et al: Influenza B Virus in Seals in Science - 2000
 Blanc et al: Serologic evidence of influenza A and B viruses in South American fur seals (Arctocephalus australis) in Journal of Wildlife Diseases - 2009
 Ohishi et al: Serological evidence of transmission of human influenza A and B viruses to Caspian seals (Phoca caspica) in Microbiology and Immunology - 2002