My understanding of it: Vaccines are not force fields that prevent you from contracting a disease. They just lower your chances considerably. So an unvaccinate

My understanding of it: Vaccines are not force fields that prevent you from contracting a disease. They just lower your chances considerably. So an unvaccinated child can infect vaccinated children. Vaccinating "eradicates" disease in that it lowers the overall chance of transmission to the point where the disease can't effectively spread. A bunch of unvaccinated kids raises that overall chance back up and can cause outbreaks.