Polio was eradicated from the US many years ago. Today, it is just one of the often forgotten vaccines and shots routinely given to children. They are given during a baby's scheduled checkup.
In Congo and most developing countries, babies do not have checkup schedules. Most babies are not even born in a health facility. Many times, health facilities and trained personnel are not available. So, it is not uncommon that poor parents do not routinely take their children to the doctors for checkups.
Sadly, that's why one out of five children in the Congo do not live long enough to see their fifth birthday. Polio kills and cripples.