Health officials say the number of U.S. measles cases this year has risen to 141.
Eighteen of the 20 new cases reported Tuesday stem from outbreaks at Disneyland in California and an Illinois day care center.
The Centers for Disease Control and Prevention says most of the people who got measles this year were not vaccinated. Some were children too young to get the shots.
Until recently in the U.S, it was unusual to see more than 100 measles cases a year, usually brought in from countries where the illness is more common.
Measles is usually seen in winter and early spring. It causes a fever, runny nose, cough and rash all over the body. In rare cases, it can be deadly.