What is the worst incurable disease?
Quite a few neurodegenerative conditions would meet the criteria in my humble opinion. Particularly the ones that are idiopathic, appear all of a sudden, have no modifiable (or even non-modifiable recognized) risk factors, nor adequate treatment. Think multiple system atrophy, catching people off gard, unprepared…they can literally be in perfect health and in matters of few hours be given one of the worst news possible. They will wane in every single way possible, having to be taken care of and helped with everything constantly, oftentime dying knowing how their loved ones hating having to dedicate years of their lives non-stop carying for them.
Then we also have things like toxic shock syndrome, same like the above except if lethal, the death is rather swift although very agonising and does not give the patient any real chance of accepting their condition, being sort of like struck with a lightning from the clear skies (seemingly clear). If caught early enough, however, it’s not impossible to save the life the patient, although more often than not patients remain crippled or/and disfigured.
Radiation sickness, if exposure strong enough, is probably the most excruciatingly painful and the most untreatable, the most hopeless, condition out in the whole damn galaxy. The thing is however, on the Earth, is almost literally (!) perfectly preventable, 100% of time. Don’t have artificial ionizing radiation sources and voila!