There's a lot of evidence that when living with somebody, you share much of their gut flora. Forensics teams have actually tried looking at the bacteria in a household, and identifying if somebody in that household has been in another location by looking at the bacterial signature they left behind. Married couples have a much more similar gut Flora than siblings are parents and children, even accounting for differences in diet and lifestyle. It really does seem to be an exposure thing vs a genetic or even lifestyle thing, although diet and lifestyle does directly effect gut flora. We believe that gut flora influences behavioural issues, autism, IBS, dementia. Why not diabetes? The bacteria in our gut help break down our food, I bet they do have an effect on how much sugar goes into our bloodstream how quickly, and perhaps has effects on our pancreas as well. This seems quite plausible to me. Some people have tried an interesting therapy called helminth therapy, which is basically giving a patient parasite, usually one that cannot reproduce, often those found in rats. People have tried it with some effect for individuals with Type 2 diabetes. The theory is that we evolved closely alongside rats and only recently in our hyper-sanitized state have we been without parasites. These parasites also evolved alongside us, humans and parasites both depending on each other for survival in a symbiotic relationship. Helminths were a big part of our gut flora and helminth therapy has shown interesting results for some individuals with ADHD and many chronic diseases.