The Centers for Disease Control and Prevention investigated more than a decade's worth of data on measles cases (including 2013 cases through the end of August) and found that most patients had not been vaccinated.
Measles is a respiratory disease caused by a virus, that spreads through breathing, coughing and sneezing and causes a serious rash and fever. While measles is harder to come by in the United States, the CDC notes it's still a worldwide public health problem that can be brought over to the country and spread to Americans.
Measles can also result in serious complications including