If your brain is wrecked by drinking, or certain kinds of drug addiction, you need medical (chemical) help. Going cold-turkey on drinking is actually dangerous to some people (the DT's), it leads to heart arrhythmias and seizures, and these people actually need drugs like phenytoin, Clonidine, and Benzodiazepines to even contemplate quitting. You might say it's just a matter of willpower, but the fact is that most people who really try to quit without chemical intervention fail. I don't know whether you can ontologically define it as a disease, but in a public health sense it has the effects of a disease, with the same social costs, and the same method of treatment. You can define "disease" for yourself, but I'm going to stick with the medical establishment's definition.