It is a habit. Often a deeply engraved one. But I maintain calling it a disease is shifting the responsibility away from the person.
I can catch cold, I can't catch heroin.
Most recovering alcoholics would refer to alcoholism/addiction as a spiritual malady. If you limit discussion of it to a simple "nature vs. nurture" you're ultimately going to have two sides that merely condone the behavior of the addict or alcoholic. The "nature" side will excuse the practitioner as having been, "born that way." The "nurture" side would argue that a learned behavior can be unlearned. It's been my experience with more than a few years of sobriety under my belt, that until and unless one is willing to recognize the spiritual component and dimension of the dysfunction, one will never start down the path of solid recovery.
"I can catch cold, I can't catch heroin."
One of the early founders of AA was a medical doctor who likened the physiological effects of alcoholism to an allergy. It's not something you "catch", but it's something different people react to differently. Subsequent studies of alcohlics have established that their bodies and brains respond differently to alcohol than do those who are not alcoholics, so there is a physiological difference. The question, to the best of my knowledge, remains unsettled as to whether those changes are inborn or if they develop over time with the consumption of alcohol. Certainly there are some who appear to be predisposed to alcoholism.
As the parent of two children who have a real disease (juvenile diabetes) I couldn't agree more.
Call it a crutch, character flaw, bad habit, weakness of will, whatever. But putting something in your mouth or veins or up your nose is not a disease.