Polio viruses. Coloured Transmission Electron Micrograph (TEM) of polio viruses, the causative agent of poliomyelitis (infantile paralysis). Although rounded in shape, each virus has a cubic symmetry. They are RNA picornaviruses. RNA genetic material occurs in the core of each virus (orange) surrounded by a protein coat (green). There are three types of polio viruses, type 1 being the cause of most polio epidemics. 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 or even death. Polio vaccines are available. Magnification: x80,000 at 6x7cm size.