Thanks to an effective vaccine, the polio virus has disappeared from most of the world. But, a few cases remain in areas that are among the most difficult to reach. To eradicate the virus completely, medical teams are trying to vaccinate children in three countries. The wild polio virus still exists in Afghanistan, Pakistan and Nigeria. But unrest, conflict and the extreme remoteness of these areas has kept thousands of children from receiving polio vaccines this year. Andrew Etsano works as an immunization expert for the Red Cross. He says that one of the difficulties the group faces is how often people in these areas move. They often flee economic problems, political situations or violence. Etsano says that, as a result, people take the virus from one place to another. And medical teams cannot always track the migrants in order to vaccinate the children.