Polio viruses (type 1). Coloured transmission electron micrograph (TEM) of clusters of polio viruses, the cause of poliomyelitis (infantile paralysis). There are three serotypes of the polio virus, and this is type 1 which causes most epidemics. Although rounded in shape (light blue) each virus has a cubic symmetry. They are picornaviruses. RNA genetic material occurs in the core of each virus, surrounded by a protein coat (yellow). Infection occurs through the mouth. Usually only a mild illness results; but in more serious cases the virus may attack the nervous system causing paralysis. Polio vaccines are available. Magnification: unknown.
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