Autism: why the massive increase?

It's been well publicized recently that the rates of autism among American kids has risen sharply in the last few decades. The CDC has said that the rate of autism has reached 1 in every 150 kids in this country recently, and even fours years ago was sounding alarm bells. That's quite disturbing, right?

What has happened here? It's no secret that the increasing rate of obesity is due to the more processed and junk foods many of us are eating, so what is the environmental cause of increasing autism rates? I highly doubt there is some genetic factor responsible for such a quick increase in recent times.

But, sometimes environment influences existing genetic predispositions to things like cancer and other diseases and disorders. Is that the case here? One thing is for certain -- the autism rate is really large, and with the future citizens and leaders of the country at stake, finding causes to things like this seems like it should be a national priority.