I agree with someone upthread, it probably is now a modern day disease caused by our obesogenic society and lifestyles.
I am slim and no-one of my family (or dh's) have been obese, I have photos of family members stretching back to the 1800's and none are overweight. However, sadly my DS21 is 8 stone overweight. He was slim as a child and right up until he started to drive and work and then the weight started piling on because he was skipping dinner at home and going out for meals, eating endless UPFs and takeaways and quick to eat food that takes little imagination and cooking.
The fact he was slim for the first 17 years of his life shows it's obviously not a disease in the traditional sense but lifestyle choices and a constant push from the food (if you can call most of the stuff food) industry for people to snack constantly.
But all of our relatives in those black and white family photos could easily have become 8 stone overweight like my poor ds in the here and now but they lived in a time where they used their bodies to move around more, cycled/walked to work, did manual work and housework, only ate 3 meals per day and didn't snack out/eat out because most people ate to live, not the other way around. There was also usually someone at home cooking meals from scratch and no mad dash to ping something in the microwave once everyone had crashed home exhausted after long days at work.
We all know that many foods these days is full of crap and is only adding to the crisis and I try not to be a conspiracy nut but the shite in food produced by the major manufacturers is highly addictive and that's got to be to keep those obese hooked on those foods wanting more....it's a legal drug and many are in desperate need of UFP rehab.
I worry so much about my son especially as he has a very fatty liver so whether his current obese state is an illness/disease or not it soon will be if he doesn't do anything about it.