It looks like it's just their blood. Apparently even a small amount can heal someone and bring them back from the dead. From this, it would make sense that they would also live forever. Just a small amount of blood heals a huge amount of damage so maybe the blood is replicating itself or directing the other cells. I'm probably looking into this too much plus I doubt the writers want to make it possible for other people to be immortal just because they got some blood.
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The black wind howls...
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