A measles outbreak centered around Disneyland in California has spread to six more US states and Mexico, and an international visitor to the theme park likely sparked the health alert, say officials.
Fifty-one confirmed cases of measles have been reported to the Centers for Disease Control and Prevention since late December, the US government agency said in a statement, most in California but others as far afield as Nebraska and Washington states.
The CDC said those who had fallen ill were aged from 10 months to 57 years and only a tiny fraction were vaccinated against measles, in the face of an anti-vaccination trend that has emerged in recent years, particularly in North America.
Opponents fear the MMR (measles, mumps, rubella) vaccine causes autism, even though an array of studies have ruled out any link.
Measles has been officially eradicated from the United States since 2000 while remaining widespread in other regions including Europe, Africa and Asia.
"In addition to the US cases, one case was reported from Mexico in an unvaccinated child who visited Disneyland Resort Theme Parks on December 17 and December 20, 2014," the CDC said.
While health officials have yet to isolate the source of the outbreak, "it is likely that a traveler (or more than one traveler) who was infected with measles overseas visited one or both of the Disney parks in December during their infectious period," the CDC added.
Most -- 42 -- of the 51 cases are in California, but three more have been found in Utah, two in Washington, and one each in Oregon, Colorado, Nebraska and Arizona.
Measles is highly contagious and can be spread through the air without physical contact. Infection usually begins with a fever followed by a cough, runny nose, conjunctivitis and a rash.
Complications can include blindness, hearing loss, pneumonia and death. One to two children of every 1,000 infected with measles will die from it, the CDC said.
Eradication means the disease is no longer native to the United States, but there were 644 measles cases in the US last year, an enormous jump from 173 cases in 2013.
An analysis by the Los Angeles Times last year found that 9.5 percent of kindergarten children in an Orange County school district were exempted from vaccinations because of personal beliefs.