In a rash of cases that public health officials are rushing to contain, at least 70 people in six states and Mexico have fallen ill since mid-December, most of them from California. The vast majority of those who got sick had not gotten the measles-mumps-rubella, or MMR, vaccine.
While still a scourge in many corners of the world, measles has been all but eradicated in the U.S. since 2000 because of vaccinations. But the virus has made a comeback in recent years, in part because of people obtaining personal belief exemptions from rules that say children must get their shots to enrol in school.
Others have delayed getting their children vaccinated because they still believe now-discredited research linking the measles vaccine to autism.
"Some people are just incredibly selfish" by skipping shots, said Dr. James Cherry, a pediatric disease expert at the University of California, Los Angeles.
As cases mount, several newspapers have criticized the anti-vaccine movement.
Measles "is a disease that has been beaten by modern medicine. That makes it all the more frustrating that anti-science stubbornness has proven, in the case of the Disneyland-related measles, that when it comes to contagious diseases, it's a small world after all," the Los Angeles Times said in an editorial last week.
Barbara Loe Fisher, director of the National Vaccine Information Center, a Virginia-based non-profit that favours letting parents decide whether to vaccinate, said, "I don't think it's wise or responsible to blame" unvaccinated people for the Disney outbreak. She noted that a small number of those stricken had been fully vaccinated.
Health authorities believe the outbreak was triggered by a measles-stricken visitor to one of the Disney parks who brought the virus from abroad last month.
As one of the world's biggest tourist destinations, Disney was a perfect spot for the virus to spread, with large numbers of babies too young to be vaccinated and lots of visitors from countries that do not require measles shots. The disease has since spread beyond Disneyland.
The infected ranged from 7 months to 70 years old, including five Disneyland workers.
"It's tragic to see measles making a resurgence," said Deanne Thompson, a spokeswoman for the Orange County Health Care Agency. "When our immunity falls, it creates a problem for the whole community."
While all states require certain vaccinations for schoolchildren, parents in certain states such as California can opt out if they sign a personal belief waiver.
In the past five years, the percentage of kindergartners in California who are up to date on all vaccinations has held pretty steady from 90.7 per cent in the 2010-11 school year to 90.4 per cent in 2014-15. But there are some wealthy communities in Los Angeles and Orange counties and in Northern California with double-digit vaccination exemption rates.
To control this latest outbreak, those who are not vaccinated were warned this week to stay away from Disney theme parks. Disney employees who have no proof of immunization and may have come into contact with sick colleagues were placed on paid leave until they are given the medical all-clear.
At Huntington Beach High School in Orange County, two dozen unvaccinated students were ordered home until the three-week incubation period is up.
More than 30 babies in Northern California's Alameda County have been placed in home isolation after possible exposure.
"I'm terribly upset that someone has made a choice that not only affects their child but other people's children," said Jennifer Simon, whose 6-month-old daughter, Livia, was isolated after it was learned she may have been exposed to measles during a visit to the doctor's office.
Contact Alicia Chang at http://twitter.com/SciWriAlicia