At least nine people – including six California children – contracted measles after visiting Disneyland Resort in mid-December, state health officials said Wednesday.

Public health officials are also investigating three additional cases of suspected measles, which is highly contagious, and a spokesman with the department said it would not be surprising to see more cases sprout up. All 12 reported visiting Disneyland or Disney California Adventure Park between Dec. 15 and Dec. 20.

An infected park-goer likely spread the airborne virus, officials said.

The introduction of a vaccine in the 1960s helped eliminate measles in the United States, which used to sicken up to 4 million Americans each year. Though uncommon, outbreaks still happen when international visitors and unvaccinated Americans become infected while abroad, then bring it back to the states, according to the Centers for Disease Control and Prevention.

Of the nine confirmed cases linked to Disneyland, including two from Utah, only one was vaccinated, according to the California Department of Public Health. Two were too young to be vaccinated. Children generally get their first dose of the MMR (measles, mumps, rubella) vaccine beginning at age 1.

Measles causes fever, cough, runny nose, red eyes and red rashes. People can be infectious with measles for nine days. They are usually contagious from about four days before a rash appears to four days after.

The seven California residents who fell ill are from Orange, Riverside, San Diego and Alameda counties, as well as the city of Pasadena. Six were children and three were hospitalized.

State health officer Dr. Ron Chapman said the vaccine is the best way to prevent measles.

Disneyland’s Chief Medical Officer Dr. Pamela Hymel said she was working with the health department to “provide any information and assistance we can.”

Staff writer Joseph Pimentel contributed to this report.

Contact the writer: jchandler@ocregister.com and @jennakchandler on Twitter