The answer to this was explained to me many years ago by a very good doctor. He explained that the problem with smoking isn’t that it causes many of the problems most often associated with it (most notably cancer), but that it exacerbates them in likely sufferers.
It’s likely that people’s predisposition to those conditions is genetic, in their DNA. Some people have no genetic predisposition to getting cancer for example. Any of those people might smoke and smoke for many years with few ill effects at all.
Having a predisposition doesn’t mean anyone will definitely contract cancer. However…
If a person with a genetic predisposition to contracting cancer is a smoker, the likelihood of them contracting it is much higher.
Now, smoking is associated with many more health problems than just cancer – think emphysema, heart disease and many others. If a person has a genetic predisposition toward contracting any of these conditions, as a smoker their likelihood of contracting them is much higher.
The only certainties in life are death and taxes, But to postpone death for as long as possible, stay away from cigarettes.

He never quit, but he certainly smoked less. He was a popular General Practitioner, but he also loved smoking, and although he said to his patients that they should not smoke, he also reminded them about the pleasures in life.
When he was 52, one of his greatest pleasures in life produced a small bump, and the bump turned out to be a messenger, and Arnold — as an MD — knew the messenger all too well.
The lung cancer’d had too much time to wiggle, and the elbow room of procrastinated smoking had given it the time to spread. He ended up in a wheelchair, because the cancer ate his spine. (It was near Christmas, if I remember well.)
The so many who still “survive” after 45–65 years of smoking, hardly do it like you imagine it, though.
My friend Dominic’s dad is one example, but there are so many others.
The damage you do as a smoker, or a drinker, or a sun bather in the first (one, two or three) decades of your life, will not be re-set in hindsight. Even one bad afternoon of sunbathing when you were twelve (the bad burns, the skin) can yield skin cancer 30 years later.
In Dominic’s father case, it was a life-long of smoking that did the deed. Sometimes it’s one single sun burn though. The last time we met, Dominic was smoking himself, and something irrational came over me — typically me — and I recalled his dad, the wheelchair, the broken spine.
And I wanted him to stop smoking, just for the sake of his dad. (We talked hours and hours, and he smoked some more, but his intentions were fine. His dad was not.)
Tomorrow, Dominic will be in my house again.
Nobody will smoke, but we will most definitely drink —
This is to the father. (This is to the son.)