Exquisite Blood

Oracle Text

Whenever an opponent loses life, you gain that much life.

Card Rulings

5/1/2012 Damage dealt to an opponent usually causes that opponent to lose life. An opponent paying life also causes loss of life.
5/1/2012 If you and an opponent both lose life simultaneously, and this causes your life total to become 0 or less, you’ll lose the game before Exquisite Blood’s triggered ability can resolve.