Measles is a highly-transmissible viral infection characterized by severe rash, high fever, cough, malaise, runny nose, and red eyes. Infections usually last 7-10 days.
The United States is experiencing a resurgence of measles, as shown in chart below (source, CDC):
A measles outbreak began in December 2014 at Disneyland in Anaheim, California. It was likely spread by air and contaminated surfaces. Forty-two people were infected at two Disney parks, including five employees. As of this month, there have been 59 confirmed cases in California. The outbreak has also spread to Utah, Washington, Colorado, Oregon, and Mexico.
The outbreak primarily affected non-vaccinated people. Orange County, California, the epicenter of the outbreak, has a particularly high rate of voluntary non-vaccination. The outbreak would probably have been prevented entirely had more people been vaccinated.