WMG / The Man Who Killed Don Quixote

Don Quixote is a supernatural entity.

This theory exposits that Javier Sanchez, the old man who began to believe that he himself was Don Quixote, did not go mad from old age or anything else, but rather some memetic, supernatural... thing irrevocably linked to the story possessed him, having either become the vessel of this spirit because he reenacts the character and it made him susceptible to possession.

There are hints that this could be the case as Toby slowly begins to experience dreams, visions and peculiar coincidences that imply that he is reliving Don Quixote's life, the process of this so called "delusion" being both contagious and slowly germinates. When Javier dies, he remembers his old self moments before he passes on, and by the next morning Toby suddenly believes that he is Don Quixote. It could be possible that those chosen by the current Don Quixote to be Sancho become the next Don Quixote. If there is no Sancho to take up the mantle before Don Quixote dies, the spirit (or whatever it is) waits until it finds a new inheritor itself like with Javier. It is likely that when Toby dies, Angelica will be its next inhabitant.

In-fact, its possible that the book itself is based off of a previous inheritor of this curse, the author Miguel de Cervantes either having known someone who thought he was Don Quixote and recorded his ravings and antics or had inherited it himself.