Dark Eden by Patrick Carman

Fifteen-year-old Will Besting is sent by his doctor to Fort Eden, an institution meant to help patients suffering from crippling phobias. Once there, Will and six other teenagers take turns in mysterious fear chambers and confront their worst nightmares—with the help of the group facilitator, Rainsford, an enigmatic guide. When the patients emerge from the chamber, they feel emboldened by the previous night's experiences. But each person soon discovers strange, unexplained aches and pains. . . . What is really happening to the seven teens trapped in this dark Eden?

Patrick Carman's Dark Eden is a provocative exploration of fear, betrayal, memory, and— ultimately—immortality.