There was a theory about thiomersal preservative in the vaccines being the cause, but that's pretty much been exploded (thiomersal stopped, but autism didn't).
But the double-whammy effect (there's a proper name for it but I can't remember it, sorry) where a child is vaccinated, shows autistic symptoms, recovers, gets vaccinated and AGAIN shows autistic symptoms convinced me that the vaccines are the disease. Iatrogenic disease didn't stop with the infamous Salk vaccine.
I must say I never thought to ask about unvaccinated autism. It just goes to show that there's no such thing as a stupid question.
Hmm, the article seems to think that thimerosal is still being used in pediatric vaccination (in the US). I'm not in a position to judge, but another Freeper must know.
The rate off autism has gone down in CA since thimerosal was removed. Trace amounts of the substance remain in all vaccines, and we still do not know how much is safe. A recent NIH study found thimerosal to be much more toxic than "regular" mercury.