This is a topic of great debate. Currently, researchers are working very diligently to identify the cause of autism. Many people have speculated about specific causes, but none have held up to scientific scrutiny.
What we do know is this: Genetics play a major role in most cases of autism. In fact, it is becoming accepted that autism is a "polygenomial" disorder, meaning that several genes may be involved, and that there may be different types of autism, depending on what genes are affected and in what way.
Most credible researchers will also concede that genes alone are probably not the sole cause of autism. Many researchers today propose the "Two-hit" theory, which basically surmises that autism is the result of a combination of genetic factors that create a predisposition to be susceptible to a certain environmental insult, and some type of environmental insult, which is currently the elusive "X-factor." This environmental piece is yet unknown and could range from exposure to some type of agent or chemical to some type of injury to the developing child.
To make any assumptions regarding what the environmental factors are that play a role in autism would be premature as valid scientific evidence has not yet been produced to support any particular environmental theory of causation.