Disease as defined by websters
: an illness that affects a person, animal, or plant : a condition that prevents the body or mind from working normally
: a problem that a person, group, organization, or society has and cannot stop
I think addiction of any kind fits the first definition, and I think can feel like the second definition from the addicts pov. Withdraw alone can kill when it comes to alcohol, benzo's, and barbiturates if proper medical attention is not provided during the process. Many studies have shown a 50-60% genetic predisposition to addiction, much like genetics predispose many to diseases. However predisposition is not destiny, and steps can be taken to lessen the likelihood. Nothing about it being a disease strips an addict of choice, at least no less than having copd strips a smoker of the choice to not smoke, or someone with diabetes the choice to not eat sugary, fatty foods, or the choice to start exercising. The reason people don't want to view addiction as a disease is it humanizes the inflicted and makes people feel guilty for looking down on, stigmatizing them, and alienating them.
Take Portugal who decriminalized most drugs in 2001, viewing drugs and addiction as health related, not criminal. HIV among injection drug users has gone from 1016 in 2001 to 56 in 2012, and new cases of HIV from injection drug users has gone from 568 in 2001 to 38 in 2012. Going from 1 of the highest new HIV infections among injecting drug users to one of the lowest among nations with this issue. If this trend continues they could have one of the lowest rates in Europe soon. Similar trends are scene for other communicable disease in needle users in Portugal. OD's have gone from 80 in 2001 to 16 in 2012. While Homicides have increased by 40% from 2001 to 2006 they do not keep track of drug related homicides, and homicides statistics in Portugal include manslaughter, infanticide, and voluntary euthanasia none of these can be absolutely tied with drugs use, unlike say shooting a man for his wallet to go buy drugs. The rise could also be attributed to drug trafficking as dealing is still illegal and if there's one thing the black market has proven time and time again is life has no little to no value. This figure comes from the United Nations Office on Drugs and Crime and they did not implicate decriminalization in this rise of homicide. Also homicides went up in Ireland, Cyprus, Luxembourg, Sweden, and Scotland during this time period, where drugs are still illegal, so it's hard to say if it is or isn't drug related. However by 2012 homicide rates have reduced back down by roughly what it was in 2002, I do not have this figure. Opportunistic crimes like theft and robberies have gone up when measured in 2004, but typical drug addict crimes like robbing of homes and businesses has reduced in the same time period. The number of drug related offenders (crime committed under the influence and/or to fund drug consumption) has declined from 44% in 1999 to 21% in 2012. During the times crimes were up since decriminalization there was also plenty of economic crisis and turbulence in all of Europe and this rises crimes regardless of addiction. Frequent drug use has steadily fallen from 50% to 30% in adults and teen drug use has fallen and risen and fallen and risen, however less teens in Portugal per capi have smoked pot than teens per ca pita have used cocaine in America. Life time drug use (people who use but not often) has risen slightly but I personally see nothing wrong with that.. The number of addicts who seek treatment in Portugal has sky rocketed, because unlike America they aren't viewed as lepers by society, and criminals by the system.
Not all of these statistics can be attributed to decriminalized but it had an effect. Moral of the story is treating drug use as a criminal and moral problem has produced no positive impact, treating it as a health issue has had positive results. Despite everyone saying Portugal would quickly become a drug mecca of murder, street orgies, senseless violence, and total degradation to anarchy. While there was an immediate rise in drug use after decriminalization it quickly tapered off and has not reached the same level since.
My 2 cents, it's more a disease than a moral flaw, or criminal act.