a good example is what happened at Disneyland with measles. A bunch of people thought their kids were immune or something. So they didn't get their kids vaccinated. Then took their kid with measles to the happiest place on earth and infected a bunch of other people who thought they were immune and didn't need a vaccine.
You can Google it for the full story. The number of infected was not small, and spread across 3 or 4 states.