As the global effort to eradicate polio gets tantalizing close to its goal, the program is running in to new challenges. One of the biggest obstacles this year is the proliferation of so-called "vaccine-derived" polio outbreaks. Conventional polio caused by the traditional form of the disease is now only occurring in two countries in the world — Afghanistan and Pakistan. The World Health Organization calls this form "wild" polio and there've been roughly 100 cases so far this year. This is a tiny number compared to the 350,000 cases that occurred globally before the Global Polio Eradication Initiative was launched...