Polio is mostly seen in third world countries like Afganistan,Nigeria,and Pakistan. Most of them are poorly sanitized and there is no cure for the disease so they can't get rid of it and it is highly contagious.
The only defense we have against Polio are vaccines to prevent people from getting it but there is no cure. If children are vaccinated from a young age and they keep getting vaccinated multiple times they can be protected for life.