Measles is a childhood infection caused by a virus. Once quite common, measles can now almost always be prevented with a vaccine. Also called rubeola, measles spreads easily and can be serious and even fatal for small children. While death rates have been falling worldwide as more children receive the measles vaccine, the disease still kills more than 200,000 people a year, mostly children. As a result of high vaccination rates in general, measles hasn't been widespread in the United States in about two decades. Most recent measles cases in the U.S. originated outside the country and occurred in people who were unvaccinated or who didn't know whether or not they had been vaccinated.