Flavorwise I would think that longterm wasting diseases such as most cancers, insanity, and certain permenant crippling injuries would be perceived as the kind of things easily cured by a "real living saint" -- therefore they would fall under Heal.
I would be inclined to restrict Cure Disease to those things that are communicable diseases.
BTW, it came up in gameplay whether alcoholism could be cured. Our DM ruled that Cure Disease would relieve the deleterious physical symptoms, but the habit of mind remained so the person would easily fall into the habit again if the opportunity arose.