Prior to 1963, when the measles, mumps, and rubella (MMR) vaccine was licensed, an estimated 3 to 4 million people, mostly children, contracted measles each year. Since then, massive immunization campaigns resulted in the successful elimination of measles in the United States in 2000.1 However, the subsequent 20 years witnessed a backslide. A small number of parents, more concerned about immunization risks than diseases they had never seen, began to refuse immunization for their children. As a result, geographic clusters of unvaccinated children are now fueling ongoing outbreaks of measles and other vaccine-preventable diseases.2