Adenoviruses are just scraping the surface, too. We have very little knowledge so far on how viruses, bacteria, fungi, and parasite animals can impact epigenetic regulation and deregulation in ways that are not immediately obvious. And obesity isn't exactly something that shows up in measurable quantities a few days after initial exposure. You wait decades, and the difference attributable to the variable under study might be a 2% difference in body fat. That's something you would only find if you already sort of knew where to look.

We're only finding this stuff now because the problem has grown to the point where researchers are specifically asking "why the heck are we so much fatter now?" They are finding multiple answers.

It turns out that real viruses can hack your body and change your persistent settings, just like software viruses can get into your computer and install a botnet rootkit. Some of those settings aren't even accessible to users. (I know if I had a slider bar to change my own body's preference for fat percentage, I'd drag it to 20% and tap the lock icon.)

Learning about this stuff, I now look at fictional plot devices like the super soldier procedure from "Captain America" in a new way. The machine demethylated all the DNA in Steve's body, and the serum injection targeted some specific genes to be re-regulated. Rather than chopping up the genes as CRISPR/Cas9 would do, an epigenetic regulator nondestructively tuned them, and set up positive feedback loops to keep them tuned to the "maximize Nazi-punching" profile. That's still far beyond our current abilities, but it makes it all seem more plausible.

Tangentially, I remember reading a paper that was linking an auto-immune reaction to some hormone-like protein produced by e. coli and both anorexia and bulimia. Are you familiar with the subject? Has it been confirmed?