I'm aware of quite a few conditions that can make losing or gaining weight harder (like thyroid issues, having a slow metabolism, having a large appetite), but I am unaware of any diseases that make a person fat. Fat is simply the difference between calories used and calories consumed, and by simply being alive you are burning calories, so it stands to reason that if you eat less than you burn, you will lose weight (or if you eat exactly what you burn, you will stay the same weight).

It's theoretically possible that you could eat more than you burn and not gain weight if your body has an issue with absorbing calories, but I don't know if this is a thing that actually happens or not.

I won't ask what your condition is out of respect for your privacy, but if you have an example of something that goes against my understanding I'd love to read more about it.