There were 222 cases of measles, a large jump from the 60 or so seen in a typical year.
The CDC says the highly contagious virus is usually introduced by people who have travelled internationally.
Children in the U.S. have been getting vaccinated for the last 50 years. Those who have not been vaccinated face a much higher risk.
Measles causes a fever, runny nose, cough and rash all over the body. In rare cases, it can be deadly.