It is not something someone has. It is not a lesion, or a seizure. Addiction is not involuntary. This distinction is important because since addiction is not a disease, it can be neither diagnosed or treated. Smoking is a behavior.
Cancer is a disease.
Drinking is a behavior. Cirrhosis of the liver is a disease. Smoking is not cancer. Drinking is not cirrhosis. Scientists abide by certain rules to identify and treat diseases.