Diseases that were largely eradicated in the U.S. a generation ago - including whooping cough, measles, mumps - are returning, in part because nervous parents are skipping their children's shots. This takes viewers around the world to track epidemics, explore the science behind vaccinations, and shed light on the risks of opting out. The vast majority of Americans - more than 90% - vaccinate their children. Yet many people have questions about the safety of vaccines.