If I were a mad scientist and could create a cure for any disease, which would I cure?
Since the prompt includes the word "mad" in the label, I am going to answer with a crazy answer; old age. I would treat old age as a disease and find a cure for it. That doesn't necessarily mean that we wouldn't all pass away eventually but we wouldn't suffer from the ravages of old age ever again. We would all die from other ways but we would die not from any effects that come from old age.
Now onto a more serious treatment of the topic. If I could create a cure for a disease which one to pick? There is Alzheimer's, Parkinson's, M.S; I am not going to list them all, but I would probably concentrate all of my efforts on the diseases that are neurologically based in the hopes that a cure for one can lead to a cure for all.
Moreover I would, since I was "mad", disseminate the cure for free to the world and drive the pharmaceutical companies crazy when they see their precious profits go up in smoke. If you are going to be "mad" might as well be "mad" for the benefit of the world.