Genetics 102: "Autism ain't a genetic disease!" - Not!
Proponents of the myriad autism causation hypotheses - mercury, vaccines, gluten, casein, plasticizers, etc. - all insist that exposure to these substances cause autism in a subset of children. None of them has said (or even implied) that any of these substances causes autism in every child who is exposed to them. The reason they don't (and can't) say that is that it would be painfully easy to refute such a claim. It is obvious that the majority of children exposed to these substances do not develop autism - since even the highest estimates of autism prevalence can only claim that a third of a percent of children are autistic. This leaves 99.7% of children - most of whom have the same exposures as the autistic children - who are not autistic.
So, how do we explain how exposure to a substance (be it a chemical, a virus or a protein) can cause no illness in most people and yet cause illness in a few? Well, the answer to that problem is genetic variation. How is it that a small percentage of people exposed to Coxsackie virus B4 develop diabetes and most people do not? Genetic variation. How is it that, in the pre-antibiotic days, Plague was not 100% fatal? Genetic variation. How is it that some groups of people are prone to breast cancer and others are prone to gastric cancer? Genetic variation.
As unpalatable as it may be to some of the autism advocates, any claim that exposure to a substance causes autism has to explain how that exposure doesn't cause autism in 100% of children exposed to it - and the answer is... genetics.
Personally, I'm fine with the idea that there are a group of children who are genetically susceptible to develop autism after exposure to some substance or another. This would be completely in line with many other disorders. In fact, it could probably be argued (and successfully) that all disease is the result of some aspect of the genome. You could even make an argument that certain accidental and traumatic injuries are the result of risk-taking behavior that might also be encoded in the genes.
So, as painful and uncharitable as it may seem, autism is - in all probability - a genetic disease. Like so many others. To deny this is to take yet another step away from reason and toward...the Dark Side.