Influenza viruses. Coloured transmission electron micrograph (TEM) of influenza B virus particles. Influenza B belongs to the orthomyxovirus group of viruses. It contains a genome of 8 segments of RNA (ribonucleic acid) genetic material inside a protein coat or capsid. Antigenic spikes (brown) on the capsid (cream) allow the virus to attach to its host cell. Influenza B infects the respiratory tract. It causes fever, aching muscles, a sore throat and weakness. New strains of the virus develop rapidly, and can cause epidemics. Magnification unknown.