I agree that a healthy lifestyle is something you have to really stick with, at least 80% of the time. I heard a saying one time that nobody gets cancer from one cigarette. It's an accumulation of cigarettes over many years that can cause cancer.
It's the same with all bad health habits. It's the accumulation of eating fast food, not getting enough exercise and being unhealthy that causes the cancer, diabetes and heart disease as time goes by.
One great thing tho is, once you change your lifestyle to become more healthy, you can heal much of the damage that's been caused and even heal illness and disease, or at leat improve it a great deal.