Nine cases of measles in two states have been linked to Disneyland in California.
Officials said seven people in California and two others in Utah have been diagnosed with the infection after visiting Disneyland or Disney California Adventure last month. Three other Californians are suspected of having measles.
All of the confirmed and suspected persons visited one of the two parks between December 15 and December 20, the California Department of Public Health (CDPH) reported.
The infected Californians are from five different cities, and range in age from eight months to 21 years old. Six of the seven were not vaccinated, and two of them were too young to receive the vaccine.
"The best way to prevent measles and its spread is to get vaccinated," CDPH Director Dr Ron Chapman said in a statement.
Walt Disney Parks and Resorts Chief Medical Officer Dr Pamela Hymel said that park officials are cooperating with the CDPH, and no staff members have reported being infected.
Health officials noted that there isn't much Disneyland could have done to prevent the transmission because measles is airborne.
US Advisory Committee on Immunization Practices Chairman Dr Jonathan L Temte said the infected were probably near an infected person who coughed or sneezed.
"If you turn around and do the math, of the people at Disneyland at the time that this occurred, probably 90 to 95 per cent were vaccinated," Temte said. "All of a sudden you realise that is a much higher attack rate."
According to the Mayo Clinic, symptoms of measles include cough, runny nose, sore throat, fever, and rash. The infection can be serious and even fatal for children.
Nonimmunised persons can be given the vaccine within 72 hours of exposure to protect against the disease. "If measles still develops, the illness usually has milder symptoms and lasts for a shorter time," thecClinic said.