The United States is on track to have its worst outbreak of measles in more than a decade. At least 89 cases have been reported in the last four months alone, compared to an annual average of around 50. Most of those are linked to a big outbreak in Europe, where more than 6,500 people across 33 countries have fallen victim to the disease. Travelers are catching the highly contagious illness while on vacation, then bringing it back to the U.S. when they return home. There have been particularly bad outbreaks in Utah, where nine children were infected after one spent time in Poland, and in Minnesota, where 21 fell ill after one child contracted the condition in Kenya. American children have been vaccinated against the disease for more than 50 years. But in Europe immunization rates have fallen over the last decade after a discredited study linked the MMR vaccine to autism, sparking the outbreak. Now health officials are warning all travelers to get the recommended two doses of vaccine before they go abroad.