A disease just means something that has a negative impact on you body. It's a super broad term, it just depends on how its used, whether its used medically or as an insult I guess.
That can't be true. That would mean things like broken bones or bee stings would be qualified as diseases too, which makes no sense.
After reading several of the posts here I still believe there's a clear distinction between diseases and injuries, and obesity falls much closer to the latter both in how it's contracted and treated.