It's called 'herd immunity'. Once you reach a certain threshold of vaccination in the population, the non-vaccinated portion is protected by the vaccinated. Problems arise when the anti-vaccination 'activists' spread lies and misinformation, effectively driving the percentage vaccinated below that threshold. Childhood diseases that should be a thing of the past are now making a comeback: measles, mumps, whooping cough, chicken pox and even polio.
http://en.wikipedia.org/wiki/Herd_immunity
Jenny McCarthy even has her own body count based on her unscientific rants against vaccines.
http://www.jennymccarthybodycount.com