JAMES M. HOGLE, HARVARD MEDICAL SCHOOL / SCIENCE PHOTO LIBRARY JAMES M. HOGLE, HARVARD MEDICAL SCHOOL / SCIENCE PHOTO LIBRARY
Polio virus. Molecular graphic of a polio virus. The polio virus is enclosed by a 12-sided protein coat or "capsid", 25-30 nanometres in diameter, consisting of repeated pentagonal units distinguished here by colour. The tiny spheres represent individual amino acids. The capsid is highly resistant to heat and acid, making the virus stable in such harsh environments as sewage systems and the human gut. Infection is by the faecal-oral route, and usually provokes no more than a mild illness. In severe cases, however, the virus attacks the brain and spinal cord, which may lead to severe paralysis or death.
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