LONDON - As doctors struggle to eradicate polio worldwide, one of their biggest problems is persuading parents to vaccinate their children. In Belgium, authorities are resorting to an extreme measure: prison sentences. Two sets of parents in Belgium were recently handed five-month prison terms for failing to vaccinate their children against polio. Each parent was also fined $8,000. "It's a pretty extraordinary case," said Dr. Ross Upshur, director of the Joint Centre for Bioethics at the University of Toronto. "The Belgians have a right to take some action against the parents, given the seriousness of polio, but the question is,...