Keeping vaccination rates consistently high over time is necessary to maintain protection. Before the development of the measles vaccine in 1963, nearly everyone in the United States got measles, and hundreds died from measles every year. In the five years before the vaccine was introduced, about 400-500 deaths and 48,000 hospitalizations from measles occurred annually. Following widespread vaccination, the United States was declared measles-free in 2000, meaning measles is not constantly present. However, outbreaks have continued to occur, with infection brought into the United States by unvaccinated travelers (Americans or foreign visitors) who get measles while they are in other countries. People with measles can easily spread it to others who are not vaccinated or otherwise protected (e.g., from past infection). In an outbreak in 2011, over 70% of cases were traced to travel abroad by U.S. citizens, nearly half of whom acquired measles in Europe. A more recent outbreak of more than 100 cases in 2014-2015 originated in Disney theme parks in California; the majority of the people affected were unvaccinated or missing doses, either by choice or because they were too young. About 20% of patients in the outbreak were hospitalized due to serious illness.