“There are different threads of concern out there” when it comes to vaccination, said Matt Zahn, the medical director for epidemiology at the Orange County Health Agency. “It becomes a game of Whack-a-Mole: As soon as you get rid of one issue, there’s another.”

The people most at risk of becoming seriously ill are babies too young to be vaccinated and the immunologically frail; measles can transform into something much worse, like encephalitis, and can be deadly. Among the fully vaccinated, the chances of contracting measles are small but do exist; the C.D.C. says the vaccine is more than 95 percent effective.

On Friday, all unvaccinated students who had been sent home from Huntington Beach High School after a possible measles exposure were allowed to return to school. But in Riverside County, officials reporting a probable case of a school employee with the measles ordered 40 students without vaccinations to stay home.

Similar scares are playing out far from Disneyland. In Kearny, Ariz., a small rural community with an economy tied to a nearby copper mine, a single family’s Christmas vacation has upended the rhythms of daily life. The family visited Disneyland in December, and four of its unvaccinated members came back with measles; a fifth person in Kearny also contracted the disease.

Now, many businesses in town — the grocery store, the post office, and more — have measles alerts in the windows featuring a blond boy with a rash all over his face. Several signs say that someone with the measles was in the store at a specific time last week and advise others who were there at the same time to be alert to symptoms.

Some parents in Kearny are sympathetic to the family that did not vaccinate children.

“I strongly believe in getting children the vaccines they need to protect them from any childhood disease out there, but that is my opinion,“ said Tiffany Magee, a mother of three. “I also strongly believe other parents have the right to choose not to get their children vaccinated due to religion or health reasons.”

Kearny school officials sent a letter home to all parents, urging calm, noting that, other than the children in the affected family, all other students in the district had been vaccinated. But it also advised parents to keep an eye out for symptoms. The measles outbreak has dominated talk in town and in two Facebook groups that discuss Kearny goings-on, and it has altered the way some people in town think about vaccines.