My point, how does the grandparent know he's never met someone with autism? Autism isn't always a visible disease. The faces of autism are the faces of everyday people. My guess is they have meet or seen an autistic person, and didn't know they were autistic because they have some kind of preconceived notion of what an autistic person looks or sounds like or acts like. Look at the pictures I posted above of my two autistic family members, if you were to see them out and about, would you think "there's some autistic kids?" Well, they aren't kids anymore, they are in their early 20s.
You're reading a lot into "seen" here. FYI, it's often used to mean "observed" or "known" generally, not necessarily in a visual or superficial sense. When addressing an argument about the supposed autism risk of vaccines, both prevalence and severity matter. If the severity of the effect is so low that these individuals do not present as autistic during normal interaction, then the risks of vaccination compared to the risks of measles/polio/etc. could be considered low despite high prevalence (which as far as anyone knows is not the case anyway).
BTW, are you aware what impression people might gain from the the vehemence and narrow focus (missing the broader point) of your response, in a discussion of anything related to the autism spectrum? Maybe you should work on reining in that behaviour instead of indulging it.
