There is no magical disease that I'm aware of that makes you fat. Retain water, yes. Bloat and slow metabolism, yes. Limit movement that slows metabolism, yes. But grow magical fat? No.
Everyone's body is different, and everyone needs to find a diet that works for him/her. (By diet, I mean 'manner of eating'.) Some people can metabolize protein very well and should have a diet high in that. Some people can process carbs better. Others need a diet almost entirely consisting of fruits and veggies with limited amounts of other things. Some people need a 'zone diet' with equal amounts of everything. Some people can eat anything they want within reason and still be healthy. Some people need over 3000 kcal to maintain their body weight; others need 1500. It's all about knowing your body, what you need, and what you can have.
And no one says that you cannot have sugar or fat or caffeine. Everything in moderation. A chai latte is cool. 10 chai lattes aren't.
Besides, fat is good for you, depending on the kind of fat. Sugar is necessary as well.
It's all about the KIND of sugar and fat and stuff you're infusing your body with, and some are truly necessary to process bodily functions and metabolize vitamins and minerals.
But if you are fat, as in more than 35% body fat, something's out of sync. And blaming it on a disease or McDonald's or stressful conditions, or shit you had to deal with growing up isn't going to solve anything.
It's your health, after all.