There is increasing evidence that differences in the kinds of bacteria which normally live in the gut (the "gut microbiome") are causally linked to Autism Spectrum Disorder.

This is currently at the basic science stage but, with more hard work it might (depending on how the results turn out), lead to new therapeutic approaches.

This (free-access but technical) paper is reasonable summary: http://www.ncbi.nlm.nih.gov/pubmed/23307560