I've thought about speciation too.  It seems very possible in the long run and is delightfully politically incorrect. I do think its possible that microbes could evolve to take advantage of a particular weakened state in humans, even if it is not coded for in the genome.  I guess this is similar to "opportunistic" pathogens. I guess fats already have increased mortality in hospital infections and for things like H1N1.  This is probably just due to immunosuppression of being so unhealthy. But obesity could plausibly be a niche that a particular pathogen could learn to target/exploit. Realistically though there could be an epidemic that we can't control, and though we might lose some elderly family members or unlucky friends who are ill or vulnerable at the time.  But the silver lining would be that probably a great majority of the fats would perish.  With a wide-scale epidemic, the health care system wouldn't be able to take care of more than a small percentage, so for the most part you'd be relying on your health standing going into it.  Plus, fats wouldn't be able to ration their food and would end up having to leave home for their beetus fix, and risk exposure to the germ.