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Influenza viruses. Coloured transmission electron micrograph (TEM) of influenza (flu) viruses. The flu virus belongs to the orthomyxovirus group of viruses which have an affinity for mucus. It contains a core of RNA (ribonucleic acid) genetic material inside a protein coat or capsid. Spikes form a fringe (green) around the capsid and allow the virus to attach to its host cell. Influenza is an infection of the respiratory tract. It causes a fever, aching muscles, a sore throat and weakness. Mutant strains of influenza sometimes appear and can cause fatal epidemics. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR LINDA STANNARD, UCT)