Nearly 300,000 health workers are fanning out across Africa this week to reach 72 million children as part of a United Nations-backed bid to drive polio out of the continent. Vaccinators will go door-to-door in 15 countries to deliver two drops of oral polio vaccine to every child under the age of five in areas considered to be at highest risk of polio, a highly infectious and sometimes fatal disease that spreads from person to person.