Addiction is no more a disease than, over-eating, homosexuality, ect.. Under the definition being used here, those things too could be considered diseases. Anyone can become dependent on drugs, that is a fact. People die all the time because of with drawls, no one is immune to drug dependency.
Is it a disease, I guess yes if you go strictly by the definition which is BS, but it's not the same as having cancer, or HIV ect. Personally I think the whole disease thing is bull shit, and also believe that the whole AA is also bull shit, and often used as an excuse for fucked up people that do fucked up shit. Some one having seizure while driving, because they have epilepsy (a real disease) and crashing, is not the same as someone getting drunk and crashing because they're an alcoholic.