From Dr. Mercola's article:http://articles.mercola.com/sites/articles/archive/2011/07/31/dr-natasha-campbell-mcbride-on-gaps-nutritional-program.aspx
It All Starts in the Gut…
Dr. Campbell is convinced that autistic children are in fact born with perfectly normal brains and perfectly normal sensory organs.
"What happens in these children [is that] they do not develop normal gut flora from birth…" she says. "Gut flora is a hugely important part of our human physiology. Recently research in Scandinavia has demonstrated that 90 percent of all cells and all genetic material in a human body is our own gut flora. We are just a shell… a habitat for this mass of microbes inside us. We ignore them at our own peril.
…As a result, their digestive system—instead of being a source of nourishment for these children—becomes a major source of toxicity. These pathogenic microbes inside their digestive tract damage the integrity of the gut wall. So all sort of toxins and microbes flood into the bloodstream of the child, and get into the brain of the child. That usually happens in the second year of life in children who were breast fed because breastfeeding provides a protection against this abnormal gut flora. In children who were not breastfed, I see the symptoms of autism developing in the first year of life.
Interview with Dr. Natasha Campbell-McBride by following the above link