This also might be a good time to ponder on something I think I mentioned before in this thread, could the obesity "epidemic" be in some way linked to our overuse of anti-biotics, creating frequent open niches in our gut flora that provide opportunities for re-colonization by the less-desirable bacteria? Basically, we did this only become a problem lately?
Another idea to consider is that these bacteria can't normally thrive because they simply run their host out of energy (as most species face pretty harsh constraints on food input because they're already living on the margins) or get their host eaten as fast as an obese gazelle in a lion cage.
If food input is constrained, it should result in a less-fit host, or perhaps less total food for the bacteria, limiting their success. Or perhaps in their normal host they serve a very useful function in helping put on winter fat for hibernation and are tuned to respond to particular types of caloric intake, and by happenstance we both picked them up and started eating in a way that somewhat matches their natural host.
There are lots of interesting evolutionary questions raised by these bugs, since it is an issue where the ecosystem in our guts interfaces with the ecosystem in the great big world of hunting and gathering.