Polio viruses. Colored Transmission Electron Micrograph (TEM) of polio viruses inside an infected cell. The spherical viruses (purple/blue) are seen among contents of a cell (yellow). These viruses are the cause of poliomyelitis (once known as infantile paralysis). The polio virus is an RNA picornavirus; it occurs as three antigenically distinct types, type 1 being the cause of most polio epidemics. Infection occurs through the mouth and transmission may be airborne. Usually just a mild illness results; but in more serious cases it may attack the nervous system causing paralysis or even death. Effective polio vaccines are available. Magnification: unknown.