If you were a mad scientist and could create a cure for any disease, which disease would you cure and why?

If I were a mad scientist, invented a cure for blindness. Somehow in the eye when you add a component the eyesight will recover gradually I think it must be terrible picture of life lost, lose or forget the color of things. For example...Get up on a sunny day, cloudy, rainy, morning or night and see the same thing; dark.