Immunizations

So I'm watching Law & Order & there's a two year old girl on here that died as a result of contracting measles from another child who's parent chose not to immunize. I have a coworker who chooses not to vaccinate her 4 & a half month old. While on the other hand my son will have his four months this month & will be getting his 4 month shots. I worry about letting her son around mine due to the fact that even though it might be rare it is still possible for her son to be carrying something that may be deadly & not show symptoms until it's too late. So my question is do you immunize your child? Why or why not?