Vaccines are one of public health's greatest triumphs. With the exception of safe water, no other health strategy, not even antibiotics, has had such a tremendous effect on reducing disease and improving health. Because of nationwide immunization efforts, the occurrence of many vaccine-preventable diseases has been reduced by more than 99% from the pre-vaccine era. However, the viruses and bacteria that cause once-common diseases such as measles, polio, tetanus still exist in the world.  Vaccine-preventable diseases cause millions of deaths worldwide and tens of thousands of death in the US each year.