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Polio, or poliomyelitis, is an acute illness brought on when one of three types of polio virus (1, 2, and 3) invades the gastro-intestinal tract. The virus has an affinity for nervous tissue, and can cause paralysis if it reaches the central nervous system. Its incubation period is 3-21 days. Routine immunisation was introduced in 1956, but people born before 1958 may not have been immunised. However, since the launch of the Global Polio Eradication Initiative in 1988, the virus is circulating in only seven countries: Afghanistan, Egypt, India, Niger, Nigeria, Pakistan and Somalia.