Having a child that has had the autism word thrown around quite a bit, really the only theory that makes sense to me is the whole gene thing. But then again we are actaully in a study that is tracking autism type behaviors through genes.
We can see how my son gets many of his traits from myself and my father. They just happen to be a bit more exagerated in him (either due to his current age, the fact he has other conditions contributing to it or something else). My mother has no autoimmune conditions at all (in fact my mother is the only member of her family that doesn't have asthma or allergies), I have asthma and allergies. None of my other cousins are even remotely near the behaviors myself and my son have shown. Further, my youngest son is completely different (I had PE with him as well). He's an extravert while myself and my oldest are introverts.
The genetics part plays a crucial role (again imo). Identical twins have between a 75-98% chance of having autism (if one identical twin does have it).
As for rates or increase in diagnosis, I look at 2 things and are very skeptical about the reliance on these increase. First, autism is more well known even in the 80's then before that. An increase in diagnosis may simply be not because it is occuring more often but because its being diagnosed more often. I was a child in the 1980's and my behaviors would have NEVER (and didn't) warrent any investigation. I was simply defined as odd in the cute child sort of way. Today, those same behaviors in my son have people screaming autism.
Second, I do worry (based on my own expereinces) about the actual diagnosis being made and where they are getting the numbers from. It is pretty well known that not all doctors giving the diagnosis go by the DSM-IV. They instead go off gut feeling or how they think an autstic child looks. Which is why the developmental Ped who tried to give my son a PDD-NOS diagnosis could not explain how he met the DSM-IV or even give examples of things like non-functional ritualistic behavior or self stimulating behavior, or even severe social deficits (especially since the entire visit my son spent the whole time trying to engage the doctor into playing with him!). I have also seen some evidence that the numbers the CDC collects do not stem from medical diagnosis but rather stem from collecting evidence from the school systems on the number of children catagorized as Educational Autism (my son has an educational autism diagnosis for his IEP and the criteria for that is not even close to the medical diagnosis), which does not require the child to have or even meet the medical diagnosis for autism.
Its these things that always make me look at the numbers and facts with a grain of salt. I had a patch of psorasis as a teenager. It lasted a whole 2-3 months and was actually brought on by stress (though I have been WAY more stressed at other times in my life and it didn't occur so there probably was a hormone issue in there as well). My husband does have psorasis that has developed as he's gotten older and my oldest son has a form of eczema on the back of his arms.