Getting away from the cancer thing, Ebola would be candidate for incurable. It’s too rare to warrant research money and too deadly for any sane person to work with.
Guinea Worm (warning - cringe-worthy reading) is nearly eradicated. Granted, it is a parasite, and the eradication is not due to medication, but due to behavior changes.
Perhaps there are other low(er) hanging fruit that can be addressed in a similarly effective way.
Not saying there is a cure, but it appears researchhas been done on it. I don’t know if there is any newer info - I’m too lazy to look.
No known diseases are going to be able to put up a fight against some hypothetical future nano-bot technology. The fact that we haven’t been able to cure some diseases so far means no more than the fact that we weren’t able to fly by gluing feathers to our arms and running really fast. Sometimes things are impossible until you get the right technology, then they’re pretty cheap and easy.
It’s quite possible that we’ll never manage to make functional nano-bots, or that we’ll destroy ourselves in the process, or that there will be other diseases that they can’t fight that we don’t even discover until we cure the known ones, so it’s not certain that we’ll be able to cure existing diseases. But we certainly can’t rule it out yet.