A measles outbreak linked to Disneyland has nearly doubled in size since last week with 45 reported cases in California and seven more illnesses confirmed in at least three other states and Mexico, health officials say.

This spread of measles is being fueled by a portion of parents who refused to vaccinate their children — an estimated one in 10 people today is perhaps susceptible to the virus, contends a contagious disease expert in California.

"The present outbreak relating to Disneyland ... is sort of the perfect storm," said Dr. James Cherry, who specializes in pediatric infectious diseases at Mattel Children’s Hospital UCLA in Los Angeles.

"People are coming to Disneyland from all over and then (you may) have one or perhaps two or three people who has measles, but didn't have the rash, and they exposed a lot of people (at the theme park)," Cherry said.

Orange County, Disneyland’s home, has the largest cluster of confirmed measles cases at 16, according to that county’s health department. In people with the serious yet preventable virus, symptoms include fever, dry cough, runny nose, watery eyes and a signature rash. People can spread the illness up to four days before the rash appears.

The initial “outbreak exposure period” has been pegged to Dec. 17 to 20 at Disneyland, say California health officials. Among the confirmed cases to date, 36 of those sick people have been linked epidemiologically to spending all or parts of those same days at the Anaheim resort, records show.

Mexican officials have confirmed one measles case in a 22-month-old, unvaccinated girl who visited the theme park Dec. 16 to 18. In Utah, there are three cases. Washington has two cases, and Colorado has one. The numbers are expected to rise. As of Monday night, at least 52 total confirmed cases were linked to people who visited Disneyland during the cited days.

Measles can lead to blindness and encephalitis, an infection of the brain. Children are typically immunized with a first dose of vaccine at 12 to 16 months and a second at 4- to 6-years-old. A significant number of Americans have opted out of getting vaccinated.

In fact, the widening outbreak coincides with new evidence that clusters of California parents have refused to vaccinate their children — decisions that boost the risk of measles hitting hard in certain towns or cities, pediatricians say.

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Researchers checked the immunization records of more than 150,000 children who are members of Kaiser Permanente Northern California — all born between 2000 and 2011, and all covered continuously by that health-care provider, which serves about 40 percent of the insured population in 13 Northern California counties.

The analysis found five residential pockets inside which vaccine refusal rates among parents ranged from 5.5 percent to 13.5 percent between 2010 and 2012. Those areas include a stretch of the East Bay area, from El Cerrito to Alameda, California.

Alameda County has had four of the state’s measles cases during this outbreak, according to state reports.

Outside of those of those five clusters, the vaccine refusal rate was 2.6 percent across the 13 Northern California counties examined, the study reported.

"You get the idea that we're now way below the level of immunization that we should have," said infectious-disease expert Cherry. In fact, some people who did receive measles shots four decades ago may have lost their immunity as time passed. "So we have about 1 in 10 people who are susceptible to measles."

That means when the exposed and unvaccinated tourists returned home from Disneyland, the virus had a better chance to get a foothold and spread in their towns, Cherry said.

"If those communities (to which the ill tourists returned) have 10 out of 100 susceptible, then you're going to have this exponential growth of measles," Cherry said. "The biggest problem is people who should be vaccinated who aren't."

Now we have people 40 years out from vaccination and so some of these people will get measles because their protection has dropped. How big a problem this is, nobody knows.

The Kaiser study, published Monday in the journal Pediatrics, also found five “significant” Northern California clusters in which children are under-immunized — that is, they missed one or more recommended vaccine doses before age 3.

"This research confirms anecdotal reports of under-immunization clusters," said Dr. Tracy A. Lieu, the study's lead author, a Kaiser Permanente pediatrician, and director of the Kaiser Permanente Division of Research.

"Our findings raise awareness that there may be communities where parents have more vaccine hesitancy and may be interested in more information or more in-depth conversations with their children's doctors."

Some parents have refused to vaccinate their children due to rumors that childhood shots cause autism — fear that was debunked research published last June. That study by the RAND Corporation reviewed all that’s known about the battery of childhood vaccines, and found them to be very safe with side effects that are very rare.

All 50 states require vaccinations for children attending public schools, but nearly every state allows exemptions to those shots. According to the Centers for Disease Control and Prevention, in Vermont, Michigan, Idaho and Oregon more than 5 percent of kindergartners had non-medical exemptions in 2013. The national average was 1.8 percent. Nationally, however, rates have been declining for many childhood vaccines.

Parents can opt out of vaccinations nearly everywhere for religious reasons and, in some states, personal belief objections also are allowed, including the stances by some parents who believe against scientific evidence that vaccines are dangerous.

The measles vaccine has been improved since the 1960s, Cherry said.

Many Americans now in their 40s, 50s were vaccinated in the 1960s. If exposed, however, some of those folks may be at risk for contracting what doctors call "secondary measles" — meaning they were vaccinated but their original immunity has, with time, "dropped below protected levels," Cherry said.

"Now we have people ... 40 years out from vaccination and so some of these people will get measles because their protection has dropped," Cherry said. "... How big a problem this is, nobody knows. But it's a real problem."