I truly believe addiction is a disease. I also think it's hereditary. Often times, the nature vs nurture is a factor but over all I think you are either born with it or not.
Many experts say 50% of children with an addicted parent will become one or marry one. Based on all the thread I have read here and seen in my own life, I think that statistic seems low, JMO.
Native American's have a high probability of alcoholism. Its a very interesting read on the reasons why! Basically, it comes done to a genetic factor and what happened when the Europeans (pilgrims) introduce alcohol to them.