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Transmission

Poliomyelitis is highly contagious. The disease is transmitted primarily via the fecal–oral route, by ingesting contaminated food or water. It is occasionally transmitted via the oral–oral route. It is seasonal in temperate climates, with peak transmission occurring in summer and autumn. These seasonal differences are far less pronounced in tropical areas. Polio is most infectious between 7 and 10 days before and after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva or feces. Virus particles can be excreted in the feces for up to six weeks.

Factors that increase the risk of polio infection include pregnancy, the very old and the very young, immune deficiency, and malnutrition. Although the virus can cross the maternal-fetal barrier during pregnancy, the fetus does not appear to be affected by either maternal infection or polio vaccination. Maternal antibodies also cross the placenta, providing passive immunity that protects the infant from polio infection during the first few months of life.