Fifty years after the development of the polio vaccine in the United States, the potentially crippling polio virus still finds refuge in some of the world's most vulnerable places, poised to re-emerge and re-infect regions where it was stamped out decades ago. The first line of defense against such a nightmarish occurrence is a vast army of health workers who go door-to-door in some of India's poorest neighborhoods, ensuring every child is vaccinated. Their mission: to eradicate polio from the planet forever.