Vaccines have saved the lives of innumerable children from infectious diseases. In the United States, state mandates for school immunization requirements and federal funding have enabled high immunization coverage and have resulted in historic low levels of many infectious diseases. However, in recent years, there have been widespread outbreaks of a number of vaccine-preventable infectious diseases, including measles, mumps, and pertussis. Reasons for these resurgences vary and are complex. They include decreases in vaccination rates, waning immunity, changes in the vaccine, and changes in the pathogen.