I'm currently trying to plot out a novel, the sequel to my Salamander and Brothers. Part of the plot involves a succession struggle. I mention in one of the previous books, set about fifteen years later, that the king took six months to die, so I need a disease that would be incurable but slow acting.
The books are fantasies in a world that is mostly like our world but with physics sufficiently different to include magic, a craft for centuries currently becoming a science. Ordinary medicine is at something like an 18th century level, so no antibiotics and no clear understanding of the nature of disease. They do, however, have magical healers. Healers have, in effect, X-ray vision, plus some ability to affect what they see, details depending on talents and training — some healers, for example, can close a wound by weaving back together the flesh. They have a very good understanding of gross anatomy, no understanding of anything at the cellular level. One very skilled healer in a previous book knows that blood has to circulate but does not know what it actually does. I don't entirely know myself, since the different physics may to some degree result in different biology. It's pretty clearly the case, although not widely realized, that all living things have some amount of magic which is in some way connected to their life.
It follows from all this that when the king, who has access to good healers, gets sick, they cannot cure him but may be able to use their abilities to ameliorate symptoms, close up sores, say. I could invent a disease but would rather use a real one, and since I don't know much about diseases I thought I would see if someone else here did and could offer a suggestion. It has to be something that would plausibly kill slowly, cannot be cured without modern medicine, ideally where the symptoms can be reduced by healers. One possibility is syphilis, which would introduce some additional plot elements, possibly but not necessarily useful, but my impression is that, treated with 18th c. medicine (mercury), it was a pretty slow killer. Tuberculosis also occurred to me.
One additional possibility. It could be that the disease is one that can kill, hence is generally believed to have killed him, but that what actually happened was that he was holding steady, with the disease not cured but symptoms held down by healers to a non-lethal level, and was actually killed by poison. _