The state is reporting 49 confirmed cases of measles so far in 2014.
Last year at this time there were just four.
This is the first year since 2000, when officials declared measles eliminated in the U.S., that the number of cases in California climbed above 40. And it's just April.
The California Department of Public Health says the highest number of cases - 21 - have been reported in Orange County. Other affected counties in Southern California include Los Angeles County with 10, Riverside with 5 and San Diego County with 4.
In Northern California the cases were spread out between Alameda, Contra Costa, Santa Clara and San Mateo.
Eleven of the patients had traveled outside the US and 30 were in known contact with a measels patient. Of the remaining cases, the patients had either had contact with an international traveler or the cause is still under investigation.
Measles is highly contagious. It spreads through the air, through a cough or sneeze by an infected person. Symptoms begin with a fever, followed by a cough, runny nose, red, watery eyes and rash. Complications can include diarrhea, ear infections and pneumonia. In severe cases death can occur.
Public health officials are urging the public to get fully vaccinated.
“Being fully vaccinated against measles does more than just protect the person who receives the vaccination," says Dr. Ron Chapman, CDPH director and state health officer. "It also protects their family and friends, including children who may be too young to be vaccinated.”