Immunizations are heralded as one of the 20th century's most cost-effective public health achievements. Immunizations protect both individuals and the larger population, such as people with immune system disorders who cannot be vaccinated. In their role as guardians of the public's health, states play a significant role in determining immunization policies. As with other public health programs, immunization programs are invisible when they are working well. High vaccination coverage rates and low incidences of diseases indicate a successful immunization program. Lawmakers face many policy issues related to vaccines and immunizations including school requirements and exemptions, vaccination financing, and vaccine safety.