Health officials say the number of measles cases in the nation is up to 121 so far this year.
All but 18 of the cases are tied to an outbreak that started at the Disneyland amusement park in California.
Until recently, it was unusual to see more than 100 cases a year in the United States. Measles are most commonly seen in winter and early spring, brought in by foreign visitors or by U.S. residents who pick up the highly contagious virus overseas.
The Centers for Disease Control and Prevention released the numbers Monday, adding 19 cases to the 102 reported as of last week.
Measles causes a fever, runny nose, cough and rash all over the body. In rare cases, it can be deadly.