Exposure works for illnesses that don't end up being life-threatening, like colds and bacterial illnesses that give us the 24 hour pukes. However, illnesses like Chicken pox, diptheria, measles, whooping cough etc. are fatal in enough kids worldwide that the more vaccinated, the better. The fact that there are so many older children and adults having the "so called childhood illnesses" is because of the generation that decided that vax was evil, and so we have diseases that were NEARLY eradicated are now popping up again, and often in near epidemic portions.
let kids be kids, let them eat dirt and kiss the dog, that works for the resistance to day-to-day germs, and for allergies too, but please, folks, have your kids vaccinated.