Cause of Autism?

My husband, who was in charge of building maintenance in a medical facility, used to say when a system broke down and the experts couldn't figure out what was wrong it always turned out to be because two completely unrelated things happened to develop problems at the same time.

We know there's an autism epidemic and many people say that's because of immunizations and mercury. But scientific studies have failed to prove that and most kids with similar exposure haven't developed autism.

What if the kids who did become autistic also had another factor? Could it be digestive problems? Some sort of hereditary condition? A germ?

Here's something I think might be a culprit; fragrances.

Over the last 30 years more and more fragrances have been added to laundry, cleaning and personal care products, cosmetics, and even foods. Because of constant exposure, most people have become unaware of the smells and "odor eliminators" also add to the chemicals in the air. But some people get more exposure to them than others, and the chemicals used vary from product to product.

What if autistic kids were exposed to more than usual of certain fragrances in infancy, prenatally, or near the time when they were inoculated against certain diseases? Could that explain at least some of the increase in autism?