The traditional difference was that diseases were caused by an outside entity (virus, bacteria, rickettsia, parasite, etc) that could be spread from one individual of a population to the next. Disorders were defined as having arose from a source that was either something wrong with the processes of the individual's body (atherosclerosis, diabetes, etc) or that was due to an non-contagious outside source (toxins, etc). At least that was the way it was explained to me in an epidemiology course.