Disease are pretty much categorized by the symptoms they cause. Because way back in the day when they were first given names, that's all we had to go by: the way someone looked or acted or felt when they something was wrong with them. But now we know what causes a great many diseases. And in some cases, the same thing - the same genetic fault, or bacteria, or virus, or whatever thing that isn't right - will be the same for a few very different diseases. Like if you get infected with streptococcus bacteria? You could get strep throat... or you could get the flesh-eating bacteria disease. So in the future, will we switch to categorizing disease by what causes them, or will we stick with symptomatic categories?
Apropos of nothing.