Very debatable. I would say that all lung cancer need not be from smoking, like all liver problem is not due to drinking. Like Komar said, I would also say (but in the reverse order) I too tried so many times to smoke (during my college days from peer pressure) but the damn smoke just made me splutter and cough and did not at all agree with me. The same goes for drinking. I just could not get it past my throat and finally used to hunt for flower pots to empty my glass.
My father died of lung cancer and he never smoked or drink... As far as I know everyone will get a cancer sooner or later. I`m a smoker and that`s one of the few pleasures that I have so I won`t quit it until it quits me :P