I've often pondered that question too. The American Medical Association sees it as a disease, yet other great medical people see it as a "symptom" of underlying psychological issues that need to be addressed.
Disease or not, it is most definitely a sickness, one that has killed too many of our loved ones. A sickness of the body, mind and spirit, I believe.
Just my thoughts, not intended to debate. When I see the difference between an active addict and that same person when they are clean, it makes me feel compassion and sadness for the pain they must be feeling.