An outbreak of measles originating in Disneyland in December 2014 that ultimately led to more than 100 cases has been attributed to parents who chose not to vaccinate their children. One year later, the United States remains vulnerable to outbreaks of vaccine-preventable diseases because parents continue to bypass the recommended childhood immunization schedule through exemptions from state-mandated immunizations at school entry. These personal choices affect everyone by weakening the herd immunity conferred by widespread vaccination.