This was an article from the New York Times 12/28/2011 about people who can't get vaccines because they have leukemia or other reasons for having a compromised immune system. They are protected from getting diseases by those who can be vaccinated, and do so.
"Young babies, the immuno-compromised and people who get chemotherapy are not able to process most vaccinations. Live vaccines in particular, like those for measles and chickenpox, can make us sick. But if 75 percent to 95 percent of the population around us is vaccinated for a particular disease, the rest are protected through what is called herd immunity. In other words, your measles vaccine protects me against the measles."