This is an attack that uses a double-sided State. Because it uses a State, it does not stack. The attack gives no immediate advantage to the person who plays it, which seems fair to me since it's potentially a very strong attack. If you do not have either state, then you take Bitten. At the start of your turn, you can trash a card costing $4 or more from your hand to get rid of Bitten, otherwise, the bite progresses to Infected. When you're Infected, you take on debt the next time you buy a card. Note, however, that since it scales with what you're buying, you can simply buy a Copper to avoid taking on debt, especially if you have extra buys, so that you can use the other buys on cards you actually want. On the other hand, if your first buy after being Infected is, say, a Colony, then you're stuck with 5D!
Cost reduction interacts with this in interesting ways. Canal and a Bridge Troll in play make Bitten more expensive to deal with, but cost reduction also reduces the amount of debt you take on if you progress to Infected. Also, if you don't buy anything at all, you remain Infected, and your opponent can't hurt you with Zombie Apocalypse. Note that the $4 cost means that a fortress in hand (assuming no Canal or Bridge Troll) effectively makes you immune to this attack. Also, this could sometimes even help you with some on-trash effects. But, most of the time it would be a fairly strong attack
EDIT: Changed the states to make them less oppressive, so now you can trash a Silver to get rid of Bitten, and even a Colony or Platinum only gives you 3 debt, with 2 debt being the maximum in a kingdom without Colony/Platinum