I Am the Salamander — Michael Friedman

Tim Cruz was the average kid. He loved the things lots of kids love. Then, one day at a baseball game, everything began to change.
He got sick. Real sick. The kind of sick people don't recover from.
So Tim did the reasonable thing. He went into a cocoon and waited for the end. But just when he thought he was toast, game over, yesterday's news . . . it turned out he wasn't. His sickness went away. Just like that.
Now he's in re-entry mode, trying to do all the things he used to do. Trying to get back into the world. But it's not easy, especially when the world's got a few surprises for him.
Tim finds himself changing -- turning into something he never would have imagined possible. Something stupid and horrifying and, frankly, kind of disgusting.
Which would be crappy enough even if he knew why it was happening. But he doesn't have a clue. And the only person who might have one has disappeared.
So all Tim can do is tell himself, over and over, as the stakes skyrocket and his life gets crazier and crazier: Don't freak out.