The California Department of Public Health is investigating nine measles cases tied to Disneyland or Disney California Adventure Park, officials said Wednesday.
The cases were reported in California and Utah and probably occurred when the victims were visiting the theme parks during the holidays from Dec. 15 to Dec. 20, officials said.
Health officials said it is likely that a single person with measles visited the parks during that period, exposing the other visitors.
"If you have symptoms, and believe you may have been exposed, please contact your healthcare provider," state health officer Ron Chapman said in a statement. "The best way to prevent measles and its spread is to get vaccinated."
The California cases were discovered in the counties of Alameda, Orange, Riverside and San Diego, as well as the city of Pasadena, and included children as young as 8 months, officials said. Two additional park visitors who were diagnosed as having measles live in Utah.
"We are working with the health department to provide any information and assistance we can, " said Dr. Pamela Hymel, the chief medical officer Walt Disney Parks and Resorts.
Two of the cases involve members of the same family from San Diego County, according to the county Health and Human Services Agency.
The two siblings may have later exposed additional people who were at the Parkway Plaza Mall in El Cajon on Dec. 29 from 10 a.m. to 2 p.m., especially those who were at the GameStop, the Sunglasses Hut or the carousel in the mall, the agency said.
Though measles had largely vanished in the United States, and was no longer considered to be a year-round threat, there has been an increase in confirmed cases.
In California, Orange County led the way last year with 22 of the 62 confirmed cases statewide. The outbreak came on the heels of a tough flu season.
County health officials expressed concern that the upswing in cases appeared to coincide with a growing anti-vaccine movement. Of the Orange County cases, many were in suburban neighborhoods where some schools showed an elevated rate of parents opting out of having their children vaccinated, or who had slowed down the vaccination process.
Though rare in the U.S., measles outbreaks continue to be reported in Southeast Asia, the Middle East and western Europe.
The vaccine, which also defends against mumps and rubella, is recommended to be administered in two doses, the first after a child turns 1, the second usually before kindergarten.
People can be infectious with measles for nine days. Measles typically begins with a fever, a cough, a runny nose and red eyes and within a few days a red rash appears, usually first on the face and then spreading downward to the rest of the body.
Measles is a highly infectious, airborne disease that is easily spread by coughing, sneezing or coming in contact with an infected person.