Several viruses are transmitted to humans through animals. But the poliovirus resides only in humans and enters the environment in the feces of someone who's infected. Poliovirus spreads primarily through the fecal-oral route, especially in areas where sanitation is inadequate.
Poliovirus can be transmitted through contaminated water and food there's some evidence that flies may spread the virus to food or through direct contact with someone infected with the virus or who has recently received an oral polio vaccine (OPV), which contains live virus. Polio is so contagious that anyone living with a recently infected person is likely to become infected too. Although people carrying the poliovirus are most contagious seven to 10 days before and after signs and symptoms appear, they can spread the virus for weeks in their feces.
Once poliovirus invades your body, it multiplies in your throat and intestinal tract and then travels to your central nervous system through your blood and lymph. As it moves along your nerve fibers, poliovirus damages or destroys the motor neurons that carry messages between your brain and your muscles.
In polio, to compensate for the missing neurons, the remaining nerve cells sprout new branching fibers (axons). Depending on how many neurons have been damaged, this regeneration may allow you to regain some or all of your muscle function, but it also places added stress on the nerve cell body, which has to nourish the additional fibers. Over time, this stress may lead to the gradual deterioration of the new fibers and eventually to the neuron itself. Researchers have theorized that this may cause the recurrence of signs and symptoms of post-polio syndrome.
The above information thankfully comes from the Mayo Clinic.com at the following link.