Is it Nature or Nurture, congenital or acquired? The answer is actually quite simple: everything is Nature and Nurture. The structure of the brain determines what we can learn and what we learn determines the structure of the brain.
The probability of having autism is greater for an individual if it runs in their family. This suggests hereditary factors. But what do you actually inherit? What you inherit is a predisposition for a specific development of your brain. This, together with what you experience while growing up, can lead to the development of a mild to severe form of autism.