It is a dis-ease in that it affects all bodily systems, but even worse it affects the brain's ability to rationally inhibit impulsive behaviors. Even at my worst, I still knew - through all the delusions - that I could get help and quit if I made that choice. So, I believe it is a dis-ease process begun by bad choices that results in uninhibited behavior due to physiological changes in the brain caused by administration of the drug.
All you can do is protect yourself. The addict is in freefall and you cannot be their parachute. They have to find the ripcord and pull it themselves.