Influenza virus. Coloured transmission electron micrograph (TEM) of an influenza ('flu) virus particle. Influenza belongs to the orthomyxovirus group of viruses. It contains a core of RNA (ribonucleic acid) genetic material inside a protein coat (capsid, white). Antigenic spikes (brown fringe) on the capsid allow the virus to attach to its host cell. Influenza is an infection of 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.