Thirteen Days to Midnight by Patrick Carman

Jacob Fielding should have died in the car accident. But right before the car slammed into the tree, his foster father whispered three words: you are indestructible. With those words a powerful force entered Jacob, saving him from certain death and dooming Mr. Fielding. But is this new indestructibility a gift or a curse? Can you really cheat death?

When Jacob tells his friends Milo and Ophelia about what happened, they experiment with transferring the power to each other and even people they don't know. Ophelia in particular becomes obsessed with cheating death, asking Jacob to transfer the indestructibility to strangers in order to protect them from what could be dangerous or deadly. But everything comes at a price, and as the days go by Ophelia begins to change. Her vibrancy turns to lifelessness, deep shadows appear under her eyes and she gets more violent and moody. Jacob and Milo have to figure this power out before they lose Ophelia - but they only way to stop her may be to give death it's due.