The words polio (grey) and myelon (marrow, indicating the spinal cord) are derived from the Greek. It is the poliomyelitis virus effect on the spinal cord that leads to the classic manifestation, paralysis. Polio reached a peak in the U.S. in 1952, with over 21,000 parallytic cases. Polio incidence fell rapidly following introduction of effecive vaccines. The last case of wild-virus polio acquired in the United States was in 1979, and global polio eradication may be achieved within the next decade. The incuation period for poliomyelitis is commonly 6-20 days with a range from 3 to 35 days. Up to 95% of all polio infections are inapparent or subclinical without symptoms. Infected persons without symptoms shed virus in the stool, and are able to transmit the virus to others.
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