Despite heroic efforts to vaccinate against this crippling disease, why does it persist? The fight to eradicate polio is an amazing story: It began with a grassroots movement in the United States and led to a global campaign to wipe out a disease that can cause paralysis and even death. There is no cure, but countless cases have been prevented by an extraordinary campaign to vaccinate every child aged five and under. It’s an operation that requires access to some of the poorest and most remote regions of the world.

But polio was supposed to have been eliminated by the year 2000. Nearly two decades later, new cases are still springing up. Why? Neal Razzell examines the challenges and the triumphs in the effort to rid the world of polio.