Nine confirmed cases of measles have been linked to Disney theme parks in California. Seven California residents and two Utah residents likely contracted the airborne illness on trips to the parks last month, according to a press release Wednesday from the California Department of Public Health.

Three more suspected cases of the highly contagious disease also visited Disneyland Park or Disney California Adventure Park in Orange County sometime between Dec. 15 and Dec. 20. People with measles can be infectious for nine days, according to Ron Chapman, director of the state’s Department of Public Health. 

“It is likely that a person infectious with measles was at one of the theme parks on these dates,” Chapman said in a statement Wednesday. “The best way to prevent measles and its spread is to get vaccinated.” Measles vaccines, which have been available in the United States since 1963, are more than 99 percent effective in preventing contagion when administered in two doses, according to the press release.

Six of the seven Californians with confirmed cases were unvaccinated for measles, and some would have been too young to receive the vaccine. The people with confirmed cases range in age from 8 months to 21 years and reside in five different regions of California: Alameda, Orange, Pasadena, Riverside and San Diego, according to state health officials.

Walt Disney Parks and Resorts’ chief medical officer, Pamela Hymel, told KTLA on Wednesday that the company was “working with the health department to provide any information and assistance we can.” There are currently no reports of Disney staff contracting measles, Disney officials told the Associated Press.

Theme parks in California attract people from all over the world. Measles was eradicated in the United States in 2000; but travelers to and from foreign countries where measles is widespread can bring the viral disease to the United States. In recent years, there have been large measles outbreaks in Western Europe, Pakistan, Vietnam and the Philippines. The first sign of measles is a fever that lasts for a couple of days, followed by a cough, runny nose, pink eye and a rash, according to health officials.