That's a theory which some believe, but there is not a shred of Scriptural evidence to back it up. Demons are evil spirits that take possession of either humans or animals. That is what the Bible reveals. They are also the spirits behind all the false gods which men worship as idols as stated in Scripture.
Going by that argument, I know people who smoke and drink alcohol and work in pollution who are well and truly alive.
The point of health laws is not primarily about health, but about accountability to God for your body temple.
The fact that some people live long even though they abuse themselves is a testimony to the efficiency of the body to fight disease, and I suppose that grants all others the freedom to abuse themselves without consequence? No, because that is how a whole nation can end up ill.
I don't care if John Smith drank himself to the age of 90, my responsibility to God is my own body, and not John's. Secondly, I cannot afford to do anything which makes my intellect diminish, because I communicate to God by it. The law "Thou shalt not kill" applies to looking after ourselves.
As for genetic predisposition, it can be improved or reduced for the next generation.