Many serious diseases are nearly eradicated thanks to vaccines. But, unless the disease is completely eliminated, it’s important to keep immunizing. For example, polio used to be a very deadly disease in the United States, crippling and killing tens of thousands of people in the 1940s and 1950s. The U.S. has been polio-free since 1979 thanks to a very effective vaccine. However, polio still exists in other countries and could be just a plane-ride away from the United States. This is why children in the United States still receive four doses of the polio vaccine.