I once visited a large cemetery plot in Fillmore County that contained the graves of eight children from one family. They were the children of a couple that had nine kids ranging in age from infancy to the teens. When diphtheria struck the household, eight of the nine died within two weeks. Only the infant survived. As I recall, this tragedy took place sometime in the early 20th century.
I have heard it argued that since we have this amazing God-given immune system that vaccines are unnecessary. I agree that I am blessed with a marvelous God-given immune system. So were these kids. The problem was that their bodies did not recognize the diphtheria germ as a threat until it was too late.
Vaccines do not replace the immune system, they enhance it. They simply warn the body what to be watching for so it can get a head start in the battle.