Diseases that are rare or nonexistent in the United States, like measles and polio, still exist in other parts of the world. Doctors continue to vaccinate against them because it’s easy to come into contact with illnesses through travel — either when Americans travel abroad or when people who aren’t properly immunized come to the United States. If immunization rates fell, a disease introduced by someone visiting from another country or returning home from a foreign country could cause serious damage in an unprotected population. In 2008, an outbreak of measles sickened many in San Diego, California, after a family who had taken a European vacation returned home with their unvaccinated child, who had contracted measles. In 2011, low immunization rates contributed to the largest whooping cough epidemic California had seen in 50 years, when more than 9,000 people contracted the illness and 10 babies died.nIt’s only safe to stop vaccinations for a particular disease when that disease has been eradicated worldwide, as in the case of smallpox.