A whooping cough outbreak in California and a measles cluster traced back to an unvaccinated child in San Diego frame this FRONTLINE investigation into the growing number of parents refusing childhood vaccinations. Pediatrician Bob Sears explains why he lets families delay or skip shots, while vaccine scientist Paul Offit describes the death threats he has received for defending the immunization schedule and dismissing any link between vaccines and autism. Public health officials walk through case files showing how quickly diseases once nearly eradicated can return once vaccination rates drop in a community. Parents on camera describe watching their children fall ill after shots and explain why no study will change their minds, set against researchers explaining how large-scale studies actually work and why anecdote and causation keep getting confused. The film does not resolve the argument so much as document it happening in real time, in doctors' offices, school board meetings, and living rooms where a family's private decision becomes a public health question for everyone nearby.