The seven Californians with confirmed cases are from five different areas of the state and range from 8 months to 21 years old. Six were not vaccinated against the disease, including two who were too young to be vaccinated.
Dr. Ron Chapman, director of the state's Department of Public Health, urged anyone who might have been exposed to check with a doctor.
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"The best way to prevent measles and its spread is to get vaccinated," he said in a statement.
According to the Centers for Disease Control and Prevention, measles is a highly contagious virus that lives in an infected person's nose and throat mucus and spreads through coughing and sneezing.
Symptoms include fever, cough, runny nose and red eyes and a red rash that usually first appears on the face and spreads to the rest of the body.
Disney officials said they had not received any reports of staff contracting measles. Park officials are working with the health department to provide any necessary information, said Dr. Pamela Hymel, chief medical officer for Walt Disney Parks and Resorts.
Health officials declared measles eliminated in the United States in 2000 because of a lack of continuous transmission of the illness.
Today, measles is brought into the country by foreign visitors or unvaccinated Americans who get the illness overseas.
Health experts said there isn't much a theme park could do to prevent transmission since measles is airborne, noting the best prevention is vaccination.
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Dr. Jonathan L. Temte, chairman of the U.S. Advisory Committee on Immunization Practices, said those who caught measles probably were a few feet from someone with the illness who coughed or sneezed—and that many others likely were exposed.
"If you turn around and do the math, of the people at Disneyland at the time that this occurred, probably 90 to 95 percent were vaccinated," Temte said. "All of a sudden you realize that is a much higher attack rate."
Dr. Alan Hinman, director of programs at the Center for Vaccine Equity, said the measles vaccine is highly effective. After the recommended two doses, he said maybe one in 100 people could still get the illness, or fewer.
"It is possible," he said. "It is not very likely."