Transmission
Poliomyelitis is highly contagious via the fecal–oral (intestinal source) and the oral–oral (oropharyngeal source) routes. In endemic areas, wild polioviruses can infect virtually the entire human population. It is seasonal in temperate climates, with peak transmission occurring in summer and autumn. These seasonal differences are far less pronounced in tropical areas. The time between first exposure and first symptoms, known as the incubation period, is usually 6 to 20 days, with a maximum range of 3 to 35 days. Virus particles are excreted in the feces for several weeks following initial infection. 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, a mode especially visible in areas with good sanitation and hygiene. 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.Factors that increase the risk of polio infection or affect the severity of the disease include immune deficiency, malnutrition, physical activity immediately following the onset of paralysis, skeletal muscle injury due to injection of vaccines or therapeutic agents, and pregnancy. 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.
So far, the only diseases completely eradicated by humankind are smallpox, declared eradicated in 1980, and rinderpest, declared eradicated in 2011. In April 2012, the World Health Assembly declared that the failure to completely eradicate polio would be a programmatic emergency for global public health, and that it "must not happen. "These efforts have hugely reduced the number of cases; from an estimated 350,000 cases in 1988 to a low of 483 cases in 2001, after which it remained at a level of about 1,000–2000 cases per year for a number of years.By 2015, polio was believed to remain naturally spreading in only two countries, Pakistan and Afghanistan, although it continued to cause outbreaks in other nearby countries due to hidden or reestablished transmission. Between 2016 and 2020 worldwide cases of wild polio (mostly in these countries) remained below 200 per year, with only 6 confirmed cases in 2021. Circulating vaccine-derived polioviruses
The oral polio vaccine, while highly effective, has the disadvantage that it comprises a live virus which has been attenuated so that it cannot cause severe illness. The vaccine virus is excreted in the stool, and in under-immunized communities it can spread from person to person. This is known as circulating vaccine-derived poliovirus (cVDPV). With prolonged transmission of this kind  the weakened virus can mutate and revert to a form that causes illness and paralysis.