Influenza viruses. Coloured transmission electron micrograph (TEM) of a section through influenza (flu) virus particles. Influenza belongs to the orthomyxovirus group of viruses. It contains a core of RNA (ribonucleic acid) genetic material inside a protein coat (capsid, green/grey). Antigenic spikes (pink) 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: x105,000 at 6x7cm size.