For fuck's sake, does anyone really want to go back to the days before immunization?

You Bet! There's thousands of 'em!

the shot caused the illness.

Evidence? None. In some individuals, immunizations do not "take" & he could have acquired the illness quite naturally, despite th immunization.

decision, that has to rest with the parents.

Not if their decision not to vaccinate causes a public health threat or endangers other children, it doesn't. That's criminal negilgence & should be prosecuted acordingly.