Type 1 diabetes—also called insulin-dependent diabetes mellitus (IDDM) or juvenile diabetes— is one of the most common and rapidly increasing autoimmune diseases in children. The U.S. has more children with type 1 diabetes than any other country in the world, with a prevalence in children and adolescents that grew by 21% from 2001 to 2009. The U.S. also has the highest number of new cases annually, well ahead of India (with a population four times bigger). Puzzling over the dramatic rise of type 1 diabetes in young children over the past several decades, scientists are reaching consensus that environmental factors play a significant role. Two of the most widely discussed environmental candidates are dietary factors and viral infections. In light of the hypothesis that viruses can be precipitating factors, it has appeared logical to at least some researchers to consider whether live virus vaccines also could be contributors, particularly given the temporal association between expansion of the childhood vaccine schedule and the escalating type 1 diabetes rates.