As many people have pointed out here, the issue is http://en.wikipedia.org/wiki/Herd_immunity http://en.wikipedia.org/wiki/Herd_immunity. Vaccines are not 100%

As many people have pointed out here, the issue is http://en.wikipedia.org/wiki/Herd_immunity http://en.wikipedia.org/wiki/Herd_immunity. Vaccines are not 100% effective, and some people can't get vaccinated (due to allergies, immunodeficiency, chemotherapy, etc). Infections spread exponentially, with each infected person infecting X other people (I'm oversimplifying here of course). The key is to make infection of a random person sufficiently unlikely that X < 1, so the infection dies out rapidly instead of becoming epidemic.