A parent's decision to not vaccinate his or her child does not happen in a vacuum. Instead, fewer vaccinated people puts everybody at increased risk of deadly outbreaks, as Aaron Carroll explains in this excellent video (via Upworthy):
What Carroll explains in this video is a concept called "herd immunity." If nobody is immunized it's insanely easy for a virus to jump from one person to another.
If a small number of people are vaccinated, that's a bit of a roadblock for the disease — but likely not an insurmountable one. There are still lots of unprotected people susceptible to the sickness. And the handful of people who are immunized aren't entirely safe, either. Vaccines do significantly reduce the risk of getting sick, but they aren't bulletproof. The flu vaccine, for example, was estimated to have a 61 percent success rate this year blocking disease.
And that mean that, if vaccines are going to work, they rely heavily "on the decreased likelihood that anyone will come into contact with the disease," Carroll says.
That's where the concept of herd immunity comes in: if enough people are vaccinated against a specific disease, then immunization does a great job at stopping it from bouncing around a given population.
That means, as Carroll puts it, "People who refuse to vaccinate themselves or their children aren't just putting themselves at risk. They're putting everybody else in danger too."
The whole video, which is above, is worth a watch — and also available here.