Jack and Hyde (The Tracings, Book One)

For most of his life, Jack Andersen has been plagued by a mysterious disease that makes him a weak, ill, and prone to injury. Over the summer, he is sent to a special hospital located in a remote desert-like area to undergo a groundbreaking treatment.

A group of doctors are supposed to be helping him get better, but in reality are keeping him sick on purpose all in the name of medical research. After months of this torture, Jack escapes from the medical facility and heads for home.

When Jack returns home, he comes face to face with an identical stranger pretending to be him.

While at first the two boys can't stand the sight of one another, they realize two heads are better than one if they have any hope of co-existing.