Cases of measles linked to an exposure at Disneyland continue to spread, not just in California, but in several other states and in Mexico. The numbers of cases are climbing — and so are the number of exposed people who might get sick — and expose more people before they realize they are sick. Measles is extremely contagious; if someone has it, they will infect 90 percent of the people around them who aren’t immunized.

It’s scary, because measles can be dangerous. 1 in 20 people who get it will get pneumonia. 1 in 1,000 will get encephalitis, a brain inflammation that can lead to seizures and brain damage. 1 or 2 in 1,000 will die.

But as scary as this outbreak is, it may ultimately be a good thing — because it may get more parents to immunize their children.

In a way, it’s our success with vaccination that is causing us problems these days. Vaccines work. They prevent the diseases they were created to prevent. And so very few people have seen measles — or polio, or diphtheria, or bacterial meningitis or even chickenpox. It’s even true of doctors; recently, some younger doctors asked me to come look at a child’s rash and see if it was chicken pox, because they’d never seen the rash themselves (it wasn’t).

When you haven’t seen these illnesses, it’s easy to think that a) they aren’t a big deal; and, b) they aren’t going to happen to your child. And if they aren’t a big deal and they aren’t going to happen to your child, why take the risk of immunizing?

The risks of immunization are actually low. In the case of the measles, mumps and rubella (MMR) vaccine, the common ones include fever, temporary joint pain, rash and swollen glands, all of which go away. The more serious side effects — seizures or a drop in the platelet count that could lead to bleeding — happen in 1 in 3000 and 1 in 30,000 doses respectively. That’s less likely than the risk of death from measles.

(The MMR vaccine does not cause autism. The one study that showed this was retracted and debunked. We’ve looked and looked at this, and there’s simply no connection.)

But these numbers aren’t low enough for some parents — especially when they have no experience with the illness. They don’t want to take any chances at all.

The Disney outbreak could be a game-changer for those of us trying protect children with immunizations. Because suddenly, there is a vaccine-preventable disease that is spreading like wildfire — and could be dangerous. Not only that, there are real and inconvenient consequences to not immunizing even if you don’t get sick. More children will be excluded from schools and daycare if they aren’t immunized. People who were exposed will be quarantined for 21 days, the length of time it takes to be sure that you aren’t going to get sick (already, some babies have been quarantined). It will no longer be easy to simply say you don’t want to be immunized — because, as has always been true, those who aren’t immunized put others at risk.

When the diseases are rare, that risk is small. When the diseases are common, the risk is large.

Now, I don’t expect any of this to convince the most ardent of the anti-vaccination people. If I can’t convince them that there really isn’t mercury in any infant vaccine, that I’m not being paid by the pharmaceutical industry, that there truly isn’t a government conspiracy (just think how many people would have to be sworn to secrecy — it’s mind-boggling) or that peer-reviewed scientific evidence trumps any one person’s opinion, no Disney outbreak will change their minds.

But the parents on the fence, the ones who are hesitant and unsure, the ones who want to do the right thing but aren’t sure what the right thing might be … those are the ones who might start vaccinating because of all these cases of measles. For them, it just might bring home what we’re trying to do with vaccines: Keep people safe and well.

That’s why, even though I’m worried about this outbreak and everyone affected by it, it may end up being a good thing. Because if more children get vaccinated against measles and other diseases, it will save lives.

Claire McCarthy is a pediatrician at Boston Children’s Hospital. She blogs at the Huffington Post, where this article originally appeared, and at Boston.com as MD Mama.