I need suggestions for a terminal illness other than untreatable cancer, specifically an illness where the "turn for the worse" could happen, and the person die within a couple of days. She needs to be at least somewhat coherent till death. I don't want to use cancer because it seems fairly common in stories like the one I'm writing. This is a minor character who has to die for the sake of the story (family visits her), but there's not a need for detailed specifics on her death. The character in question is an elderly female in present day USA.
An elderly female in the present-day USA might easily have a series of TIAs (mini-strokes) which she ignores because she improves quickly, then the big stroke which kills her in a matter of minutes or days, depending on what your story requires. Strokes are linked to uncontrolled high blood pressure, with lifestyle (both diet and exercise) contributing factors.
Plenty of older women do not exercise at all, and many refuse to change the way they've eaten all their lives in exchange for a few more years of being elderly. Many of those who would try to eat more wisely don't understand what it is they're supposed to avoid, my husband's mother being a prime example.
My mother-in-law also could not keep her various medications straight by the time she reached her 80s, and when she was particularly confused and apparently stopped taking many or all of them, her pressure soared to the highest her doctor had ever seen in any patient, 280/180. (Yeah, I know. Her head should have exploded.)
We got her meds under control, but later, she did have a number of small strokes, then one bad one which caused substantial brain damage. It could just as easily have killed her. A similar incident, or series of incidents, might serve your story just fine.
Maryn, available for questions if you have 'em