Causes of autism come from many different areas. Genetics, to environmental contaminants are a couple.  Other's have neurological developmental problems stemming from other various issues.  One of those has to do with diet and how the gut bacteria is more abundant in people who are diagnosed with autism.  https://www.autismspeaks.org/science/science-news/autism-study-more-evidence-linking-altered-gut-bacteria-asd  

 

Another is the gut-brain connection and how it is linked.  http://www.newstatesman.com/politics/health/2016/06/autism-and-gut-bacteria-surprising-link-between-mind-and-stomach