One type of immunity that doctors rely on is called “herd immunity” It assumes that because most of the population is immunized, even non immunized people are protected. The more people that refuse vaccinations increases the risk of everyone, even immunized people because vaccines aren’t magic. It is possible for a person, like me for example, to be immunized and the vaccine does not “take” for a variety of reasons.
Also, certain extremely dangerous illnesses have been eliminated from the population because of aggressive vaccination. I’m not talking about chicken pox that is relatively benign to a healthy child. Whose kid has had polio nowadays? Polio has been largely eradication because of vaccination.
Refusing vaccination will
1) make your child susceptible to infectious diseases which can be debilitating and deadly.
2) Infect others who are non vaccinated or non immunized despite vaccination
3) Precipitate the return of deadly infectious diseases.