Let me take a swing at this one:
http://www.thefreedictionary.com/disease
dis·ease (d-zz)
n.
1. A pathological condition of a part, organ, or system of an organism resulting from various causes, such as infection, genetic defect, or environmental stress, and characterized by an identifiable group of signs or symptoms.
Organ? Brain.
Resulted from: Repeated exposure to an opiate.
Characterized by: Onset of opiate withdrawl. The PHYSICAL changes in your brain that occur during opiate addiction. (op receptors changing).
http://www.emedicine.com/asp/dictionary.asp?keyword=disease
disease
Pronunciation: ( diz¢ez) 1 . An interruption, cessation, or disorder of body functions, systems, or organs. syn: morbus syn: sickness syn: illness 2 . A morbid entity characterized usually by at least two of these criteria: recognized etiologic agent(s), identifiable group of signs and symptoms, or consistent anatomic alterations.
Now, I know that admitting its a disease kind of hard for a bunch of drug addicts to do, but how the hell isn't (at least opiate) addiction disease?
The only reason I can use this solid evidence is because opiates are the most researched drug ever. If opiate addiction is DEFINITELY a disease, than I would wager, the more we learn about other drugs/addiction most all drug addiction would be a disease.
I am would wager that there is a parallel psychologically disorder that occurs in most 'drug addicts' that some people seem more likey to get than others (Take it, or leave it people).
With opiates we can manage the physical side of the disease fairly well with medication, but still people can't kick. I would say these people have more of a psychologically disorder called, for lack of a better term, drug addiction.
I guess, the term drug addiction is too broad a brush for the time being, as it can kind of describe either the disease part or the psychological part.