But jeeze, that book-keeping. There's way too much fiddling around to consider going up or down levels in the middle of a fight.
Dark Souls - the best D&D videogame - has some great alternatives.
Your max HP is halved. How simple is that? Keep track of what your max HP was, and keep increasing that as normal as you level up, but you can never have more than half of that. Season to taste. The sadist DM may wish to allow players to be double or triple cursed to 1/4th or 1/8th of HP.
Curses are generally inflicted by Basilisks, which obviously look like this:
Also known as "Curse Frogs," these guys puff up their throats and spew cursed smoke everywhere. 1, 2, or 3 saves before you are cursed, according to taste. Anyone who is cursed will immediately solidify as a jagged stone statue, returning the next morning as a shivering wreck. These statues will be littered around any place where Curse monsters live.
The Accursed are the only ones who can harm ghosts. In the middle of ghost towns, you can find men who will take away curses. Curses can never be lifted - only transferred onto someone else.
Your head sprouts egg-sacks. The creature inside will devour half the XP you earn from now on. Every time you level up, it will too, growing and hatching into... well, you're the DM. Use my worm table for inspiration if you want.
This malady is generally inflicted by the egg-burdened servants of Chaos Witch Quelaag.
They'll grab your legs, and their maggots will infest your head. If you receive their sacred burden their leader will welcome you as one of their own, and he has much sorcery to teach. They may even have a cure, though they consider the idea the vilest heresy.