Influenza B virus is the closest relative of influenza A virus. Is a genus in the virus family Orthomyxoviridae. Infect humans and seals. The Influenza B virus genome consists of eight segments of linear negative-sense, single-stranded RNA. Their capsid is enveloped while its virion consists of an envelope, a matrix protein, a nucleoprotein complex, a nucleocapsid, and a polymerase complex. It is sometimes spherical and sometimes filamentous. Influenza B virus normally exists at a lower prevalence and causes less severe disease than influenza A viruses.