EDIT: I have retracted the conclusion of this comment.

Figuring this out is basically a matter of doing a literature survey, judging a few of the studies for basic sanity, and reporting the results. Being able to do this is a very important skill, and everyone should practice it at least once so that if they suddenly develop a medical condition, they know how to double check their doctor's work. So here's what I found in the literature:

• Literature survey of multiple pathogens indicated to cause obesity
• Injecting AD-36 into rats makes them gain weight
• Injecting AD-36 into monkeys makes them gain weight
• Observational study showing AD-36 antibodies are correlated with obesity
• AD-36 correlated with greater BMI and body fat mass, but not with greater insulin resistance and again by the same department
• AD-36 makes cultured human adipose tissue accumulate more fat
• AD-36 correlated with obesity in children
• AD-36 increases glucose uptake by fat cells
• Chemical mechanism by which AD-36 induces adipogenesis

So that's correlation in both controlled animal tests and observational human tests, correlation in cultured human tissue, and observed mechanisms. A few names show up in a bunch of the studies, but there are also replications by unrelated research groups. There were even more studies to sort through, but I stopped there, because that's more than enough: being infected with AD-36 makes you fatter, proven beyond a reasonable doubt. Any commenters nitpicking the statistics and sample sizes of individual studies are missing the point, because the evidence is the sum of all the studies, and they are not all flawed, or at least not flawed badly enough to seriously undermine the conclusion.

That said, there are conclusions which it is tempting to leap to, but which I could not find support for. First, it is tempting to conclude that AD-36 infection is responsible for a large fraction of the rise in obesity in the United States. However, I could not find any data on the overall prevalence of AD-36 in the US or world population, nor on factors affecting AD-36's transmissibility; the studies I found seem to involve participants drawn from narrower geographic regions, whose exposure would be correlated and therefore not representative of the US as a whole. I also couldn't find any sort of historical data to indicate that AD-36 prevalence has risen, rather than remain constant. Not that I would expect to find historical data in any case, since that's very difficult to collect for recently discovered pathogens - but there are several inferential steps still not covered. The most likely conclusion, therefore, is that AD-36 accounts for some of the rise in obesity in the US, but an unknown amount, and not all. I also saw references to several other pathogens indicated to cause obesity in humans and animals, but did not research them in depth.