Endocrine means it secretes a hormone, leptin in this case. Didn't read the book, can't comment. But the snippet speaks of a virus that leads to excess fat accumulation/proliferation. It's possible, but it's also possible that it does it indirectly through the action of insulin. For example, if the liver is infected, it could then interfere with insulin-degrading enzyme, which in turn will prevent insulin from being degraded, then in turn cause insulin level in the blood to rise with corresponding effect on fat tissue, namely excess fat accumulation and insulin-induced lipohypertrophy.

Fat tissue doesn't convert glucose to fat exactly, instead it converts it to glycerol, which is then bound to 3 fatty acids to make triglycerides (esterification) that can't get out of fat cells cuz now they're too big to go through the cell membrane, the whole thing driven by insulin. That's the Gary Taubes basic logic for excess fat accumulation from dietary carbohydrates. Same result, just more accurate explanation of the mechanism.

Creation of fatty acids (by fatty acid synthase) does not necessarily lead to excess fat accumulation. Fatty acids are used for other things such as hormones for example. So, it depends where this creation occurs, and if I'm not mistaken it does not take place in fat tissue. It does occur in the liver, and the gut provides a lot of it from diet alone.

Creation of more fat cells is called adipocyte proliferation and differentiation. This process is mainly controlled by insulin, i.e. insulin-induced lipohypertrophy.

Having said all the above, since it's all done by signalling, and since viruses/virii have the ability to take over DNA for replication and perhaps signalling of their own, it's possible some virus can take over that particular signalling in these several places simultaneously, but I think it's much more likely that it's all done just through the action of insulin since insulin does all the above already.

This is quite interesting to me because I've been harping on about the possibility of an as-of-yet unknown underlying problem which, for example, could be quickly exposed just by going low-carb. So, if low-carb is the only problem, we go low-carb, all should get better. But, if there is an underlying problem, such as this virus they're talking about, low-carb will expose it because there will still be a problem. Also, it's possible that it will be more easily identifiable at this point because we've removed the symptoms caused by carbs, and we're left with only the symptoms of that particular pathogen/disorder/whatever.