Diseases that were largely eradicated in the United States a generation ago–including whooping cough, measles, mumps–are returning, in part because nervous parents are skipping their children's shots. Vaccines - Calling the Shots, a new NOVA special, 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 vaccinate their children, and most do it on the recommended schedule. Yet many people have questions about the safety of vaccines, and in some communities, vaccination rates have fallen below the level needed to maintain "herd immunity" –allowing outbreaks to take hold and spread. This film draws on the latest, best available evidence to help parents find the answers. Highlighting real cases and placing them in historical context, Vaccines–Calling the Shots traces outbreaks of communicable diseases and demonstrates just how fast they can spread–and how many people can fall sick–when a community's immunity barrier falls.