From a draft manuscript by our friend Dr. Kimber Rotchford:

Addictions are our human tendencies to be neurotic taken one step further into the realm of brain disease.

So, attachment, in the Buddhist sense, is psychological and social fixation on desire (neurotic), but  is also physiological, in that it is physically embodied, including but not limited to in the brain. The brain is altered by the process. At some point this physiological alteration becomes sufficient for it to be called a brain disease. That then, in Kimber’s perspective as I understand it, is the point where it is designated as addiction. Some addictions involve irreversible physiological changes and thus require ongoing drug treatment to maintain functional brain chemistry.

Notice here that all attachment, all fixated desire, has a physical element. When of  character and degree to be signficantly disfunctional (by some standard), then it is a disease, like other physical diseases.