Temple Grandin talks about "autism genetics," which is just basically nerds. Basically, people are meeting each other and making babies together who never would have met or bred in a less mobile and technological society. So they're putting their recessive genes together a lot more often now than they used to, which could account for some of the increase in autism diagnoses. (My sense is that most of that increase is simply due to better, wider diagnostic criteria and a more accepting public, and that there have always been autistic people who were simply called retarded or just weird before, but I'm no expert.)
I think if you're in the mindset that autism is necessarily a bogeyman that steals people's children, it can be alarming to think that two nerds breeding might result in an autistic child. IMO, though, autism is simply a different brain wiring that has a wide variety of outcomes, not all of them negative. Some have profound impairments, some only mild ones, and some have skills that are rarely seen in the "normal" population. But regardless, each and every autistic person is a person who deserves respect and can have a better life with good treatment.