I never get sick

Its been many many years since ive even had the slightest thing as a cold even when im living with a household full of people i wont catch the flu they have etc, at 24 this would not be uncommon but the thing is i live a horribly unhealthy lifestyle. I drink alcohol most nights mainly cheap wine, i smoke & do various drugs every week, i have a fairly poor diet mainly consisting of pasta, various meats & a few veggies I also eat my fair share of take away, the only vitamin supplement i take is about 500mcg of b12 every few days. Im not unfit or over weight if anything im under weight and probably weigh 60 kilos if not less.

I simply dont understand how for the lifestyle i live and have lived for a decade i manage to stay "healthy" it seems i have a very good immune system & the only times i ever feel drained is when i get 12 hours sleep over 4 days in the working week. Im sure one day this will all catch up on me, its just hard to want to take better care of my self when im perfectly fine not doing it.