What happens in this infinite damage/infect loop?

Question

So I have an Exquisite Blood and a Sanguine Bond in play under my control, then I attack my opponent directly for 1 damage, but the opponent has a Phyrexian Unlife and a Melira, Sylvok Outcast in play. What happens after they take the damage? Do I win?

Answer 1

Depends on how much life your opponent currently has.

If your opponent has non-positive life

1. Your opponent would lose a life from the damage except for Phyrexian Unlife.
2. Instead, your opponent would gain a poison counter from the damage expect for Melira, Sylvok Outcast.
3. Instead, nothing happens to your opponent.
4. Exquisite Blood doesn't trigger since no life was lost.
5. Sanguine Bond doesn't trigger since you didn't gain any life.
6. The game goes on.

If your opponent has positive life

1. Your opponent loses a life from the damage.
2. Exquisite Blood triggers and gives you life.
3. Sanguine Bond triggers and cause your opponent to lose a life. Phyrexian Unlife doesn't prevent or replace loss of life, so it doesn't effect this.
4. Exquisite Blood triggers and— Oh my, we've entered a loop of mandatory actions. If noone has a way of ending it, the game ends in a draw.

104.4b If a game that’s not using the limited range of influence option (including a two-player game) somehow enters a “loop” of mandatory actions, repeating a sequence of events with no way to stop, the game is a draw. Loops that contain an optional action don’t result in a draw.

Note that neither of the players are forced to end the loop even if they could. For example, if your opponent could sacrifice Phyrexian Unlife (ending the loop by losing), he doesn't have to do it.

716.5. No player can be forced to perform an action that would end a loop other than actions called for by objects involved in the loop.

Answer 2

This would be considered a "mandatory infinite loop" because the effects are recursive.

This is covered in the rules thusly:

104.4b If a game that’s not using the limited range of influence option (including a two-player game) somehow enters a “loop” of mandatory actions, repeating a sequence of events with no way to stop, the game is a draw. Loops that contain an optional action don’t result in a draw.

Now, if someone can respond to one of the triggers (even the 1,000,000th one) the game is not a draw. But if no one can, or if no one reveals that they can (it could be a Doom Blade in their hand, they are not forced to reveal they have it or anything) or simply chooses not to interrupt the loop, it will be a draw. Your opponent could, in theory, lightning bolt himself to put the game into a draw if he thought he was going to lose in a few turns, for instance.

Answer 3

No, you don't win. Sloppily spoken, what hapens instead is that either the damage is absorbed by Unlife, or the game crashes.

Let me explain:

Phyrexian Unlife prevents damage for as long as your opponent still has life left, but not the loss of life. That means if your damage manages to pass Unlife, your combo rolls out just as usual, draining life from your opponent in an infinite loop - however, since your opponent in this situation can't lose the game for having 0 or less life, this loop actually never comes to an end, and the effect cannot stop resolving.

Oficially, a situation with an unresolvable loop-up like this and no way to break out of it actually means that the game ends as a draw (see 104.4b, specifically). That's how it's handled in tournament settings.

But I've seen casual playgroups deal with these situations by ignoring the loop and just assuming that the effect has resolved for an amount of times chosen by the effect's controller, which is something making more sense gameplay-wise on my cap. Applying that to your example, you'd be free to have your effect resolve 500 times, dropping your opponent to -500 health and gaining 500 life. But yeah, since that's purely non-official, I'd recommend talking to the people you play with about how they wish to resolve a "game crash" like that in advance.