Nine people who visited Disneyland or Disneyland California Adventure Park during December have confirmed measles cases, according to California state health officials. Seven of the patients live in California and two live in Utah.

State and county health officers are investigating an additional four suspected cases, two in Utah and two in California. All the patients visited the parks in Orange County Dec 15-20, California Department of Public Health officials said.

“If you have symptoms, and believe you may have been exposed, please contact your health care provider,” Dr Ron Chapman, CDPH director and state health officer, said in a statement. “The best way to prevent measles and its spread is to get vaccinated.”

Kathleen Harriman, chief of vaccine preventable diseases for the state, said that “it’s our speculation that there was an [infected] international visitor at one of the parks, and that person or persons was able to infect a lot of people.”

Measles is very infectious because it spreads through the air, so you can catch it by, say, standing in line next to someone who is infected. Vaccination eliminated measles in the United States in 2000. “So all the cases of measles in the United States originate with an imported case,” Harriman said, “even though there can be transmission once one of those cases gets here.”

The California cases are in Alameda, Orange, Pasadena, Riverside and San Diego. The patients range in age from 8 months to 21 years. Of the seven California cases, six were not vaccinated, although two were too young to be vaccinated. The first of two recommended measles vaccine doses is typically given at 1 year of age.

Just one of the cases was fully vaccinated. Harriman said that while the measles vaccine is highly effective, conferring lifelong immunity in 99 percent of people who receive two doses, there will always be a small number of people who can get infected despite being vaccinated.