My last cigarette was February 18th 2002. That was the day I came down with a really nasty case of the flu. Very high fever, nausea, muscle aches, I was flat on my back in bed for a week. The whole time in abject misery and craving a smoke. I came out of that experience associating smoking with feeling miserable. I had smoked for over 30 years and I haven't had a cigarette since. Don't underestimate the power of negative reinforcement.