...classroom. That's not the case. Those 29 kids and the teacher are not the only people with whom he will come in contact. Measles is extremely contagious, and he risks contacting others in the halls, bathrooms, lunchroom, bus, and on the playground.

Your 1 kid among 30 at any given school might be joined by other kids in other classes who have not been vaccinated, either by choice or by some legitimate inability to do so i.e. allergies.

Next there's the idea of herd immunity. The vaccination is effective for the large percentage of people who receive it, but not for everyone. So sometimes--rarely but sometimes--a vaccinated child is still susceptible. The more people who choose not to be vaccinated, the more we risk harming those weak members of "the herd".

Kids are also notoriously bad germ spreaders. They cough and sneeze on others, and that virus can live for several hours. A teacher with an unvaccinated infant or toddler is at risk. Unvaccinated younger siblings are at risk.