How much harm can occasional smoking do?

The body CAN repair minor damage done by environmental factors.  The smoke from a cigarette would be an environmental factor.

The damage that occurs to smokers is because they never give the body a break from smoking so that the damage can be repaired.  Smoking several cigarettes per day means that the body never gets that chance to repair and rejuvenate the cells that smoking has damaged.

If something happens to you and you die and a coroner is examining your remains - he is NOT going to be able to tell that you might have smoked a few random cigarettes over a long time span.  Your body would be repairing any damage that a single cigarette or two per month (or week) might have done.

Obviously - not smoking is still the better option.  You never know what point might turn that random cigarette into a daily habit instead of an occasional thing.

The odd cigarette isn't going to kill you. I usually smoke socially when I am out with mates. Last year we went to our village open air carol service at Christmas and one of my mates opened up a pack of Mayfair during the service and she offered me one so I accepted.

No safe level of smoking has ever been established.

I've read that if you quit smoking before the age of 30 that the lasting damage is minimal. BUT I would definitely encourage stopping even a cigarette every now and again. I did the same thing when I was 18-19 (I'm 22 now). I only smoked when drinking/when it was offered. That turned into a 3 year battle trying to quit. I increased from a one or two every now and again to a 1/2 to full pack a day. I haven't had a cigarette in over 6 months. In my opinion, having a cigarette every now and again is a gamble not worth betting on.

I’m sure you are completely fine your body rejuvenated itself and you are still young