You're on a guided tour of Jerusalem, and your friend begins acting strangely. At first you think he's just jet-lagged and tired, but once he's wandering around in bed sheets and proclaiming himself John the Baptist, you know something's really wrong. Your friend has Jerusalem Syndrome.

Jerusalem is an important place to a great many people, especially followers of three of the world's major religions: Christianity, Judaism and Islam. Jerusalem is the Holy City, and pilgrims flock to it every year to come closer to the foundations of their faith. For Jews, the entire city is holy, but especially the Western Wall, which is all that remains of the great Temple destroyed by the Romans. Muslims come to the Dome of the Rock, a shrine that is the third-holiest place in the Islam faith. Christians make pilgrimages to the Church of the Holy Sepulcher, which marks the spot where Jesus is said to be crucified and buried, and the Via Dolorosa, the path Jesus is said to have traveled carrying his cross.

Jerusalem is also a highly political city, bitterly fought over by different religious and cultural factions. So in this ancient place, rich with a painful and beautiful history, perhaps it's not surprising that those who go there searching for meaning find more than what they were bargaining for. Imagine that you're a small-town girl from middle America, raised on the Bible, and you're standing at the exact same spot where Jesus, your savior, is supposed to have died. You might be disappointed -- this ordinary-looking dusty spot is what I came here to see? On the other hand, you might react with joy or awe. At this moment, you are so close to your God that it's overwhelming.

But not everyone leaves proclaiming himself or herself a prophet. Are these people mentally ill? Or are they simply overcome by a powerful spiritual experience? In this article, we'll explore Jerusalem Syndrome and look at some real-life examples of people who have it. We'll also discuss whether or not it's real, what causes it and how to treat it.