Measles, once a rite of passage for thousands of children (and their parents), has largely been eliminated in this country, thanks to the MMR vaccine. How this happened is an illustration of herd immunity: that is, that although not everyone is vaccinated against measles, enough people have so that the virus has a hard time finding a new host. However, recent, small outbreaks in the U.S. have public health officials worried. The outbreaks mean that herd immunity is breaking down. Producer Barbara Bogaev explores why even a minor Measles outbreak can be a major risk, and how some community pressures, social stigmas, and disputed medical reports have lead to the breakdown.