Officials said there was now evidence that the measles outbreak had spread beyond people who visited Disneyland between Dec. 17 and 20 and begun infecting people in the broader community.
It is the beginning of a scenario experts have feared. Health officials generally hope a measles outbreak can be contained within a manageable group of people and eventually extinguished by keeping the ill at home or in a hospital room until they recover, with the outbreak eventually being stopped by the broader community of vaccinated people.
But kindergarten measles vaccination rates have been falling almost every year since 2002 in California, and the virus now appears to be spreading.
Hardest hit has been Orange County, home to Disneyland. On Friday night, county health officials said there were six new cases of measles diagnosed among people who did not visit Disneyland before Christmas. State officials said there were two more such cases in Ventura County, and one in Alameda County.
In a statement, Orange County health officials said "the identification of six measles cases with acquisition from unknown community contacts indicates exposure to measles is more widespread throughout the county."
They added: "The Health Care Agency expects that the measles outbreak will continue to spread."
The Orange County health officer, Eric Handler, warned that students not vaccinated for measles may be excluded from attending school or day-care to prevent further spread of disease. A person who may have been contagious with measles was at Huntington Beach High School on Jan. 7-8.
There are now 16 confirmed cases in Orange County. San Diego County has 10 and Los Angeles County has eight, followed by Alameda County with four, Ventura with three, and Riverside and San Bernardino, both with two, for a total of 45 measles cases in California.
The six other confirmed cases are residents of Colorado (1), Utah (2), Washington state (2), and Mexico (1), a 22-month-old unvaccinated girl who visited Disneyland between Dec. 16 and 18.
Officials have said many of those ill were not vaccinated for measles. In the San Diego County cases alone, nine of the 10 who fell ill had not had the measles-preventing shot.
For more than a decade, measles has been considered eliminated from the United States and the rest of the Western Hemisphere, meaning domestic transmission of the disease does not occur here. Measles has still been a problem when brought in by travelers from Western Europe and Southeast Asia, but the disease has been eventually extinguished by a ring of vaccinated people.
But health officials have long expressed fears that progress against measles was threatened by a growing anti-vaccination movement in the United States, based on parents' fears that the measles vaccine causes autism -- a theory that has been thoroughly discredited by numerous scientific studies. Health officials say the measles vaccine is very safe.
"The greatest threat to the U.S. vaccination program may now come from parents' hesitancy to vaccinate their children. Although this so-called vaccine hesistancy has not become as widespread in the United States as it appears to have become in Europe, it is increasing," Dr. Mark Grabowsky, a health official with the United Nations, wrote last year in the Journal of the American Medical Association-Pediatrics. "Many measles outbreaks can be traced to people refusing to be vaccinated; a recent large measles outbreak was attributable to a church advocating the refusal of measles vaccination."
A Times analysis published last September reported that the rise in vaccine exemptions among kindergartners because of parents' personal beliefs was most prominent in wealthy coastal and mountain communities, such as southern Orange County and the Santa Monica and Malibu areas.
Health officials are urging people suspected of having the measles to first call their health provider before going to a clinic, enabling caregivers to make special preparations so patients don't risk infecting others in the waiting room. An urgent care clinic in the San Diego suburb of La Mesa was forced to shut down on Wednesday when five people arrived with the telltale rash.
Officials said that the best prevention is inoculation; the measles vaccine was introduced in 1963 in the United States. In California, two doses are required for children entering kindergarten, but parents can obtain a waiver for a child by submitting a "personal belief exemption."
Measles is one of the most contagious diseases known and can be especially severe in babies, toddlers and pregnant women, and other adults as well. Especially vulnerable are infants less than 12 months of age, who are too young to get their first dose of the vaccine, known as MMR for measles,
For every 1,000 children who get the measles, one or two will die from it, and one will get brain swelling so severe it can lead to convulsions and leave the child deaf or mentally impaired, the U.S. Centers for Disease Control and Prevention said.
A 1988-91 measles outbreak caused about 75 deaths in California alone, most involving babies and children under age 5.
Measles spreads through the respiratory droplets that become airborne during a cough or sneeze. "You can catch measles just by being in a room where a person with measles has been — even if the person is gone," the CDC says.
A person with measles is infectious as soon as coughing and sneezing begins, but before the telltale rash appears — first on the head, then spreading to the rest of the body. Patients can be contagious four days before the rash appears and four days after. Other symptoms include fever, redness of eyes and runny nose.