JERUSALEM–More than 2 million visitors flock to this holy city in a good year – including business-people, pilgrims and vacationers – and most survive the experience without suffering major upset.

But there are some notable exceptions.

In the medical literature, the condition is referred to as a form of "psychotic decompensation" or a "unique acute psychotic state."

Or, to put it in layman's terms, the City of Gold can have a strange effect on some people's heads. About 100 people a year, on average.

There's even a name for the sometimes-unnerving result.

They call it the Jerusalem syndrome.

It seems there is something about this lofty city of limestone walls, winding streets and cypress groves, a city redolent with the ancient histories of three great world religions, that makes some people go – not to put too fine a point on it – insane.

"People come to Jerusalem with deep religious convictions, and they go over the edge," says Jeremy Milgrom, a local rabbi.

In its so-called "pure" form, the phenomenon tends to follow a fairly consistent, if bizarre, pattern.

Within a day or so of arrival, some visitors to Jerusalem will gradually cease to interact with their travelling companions, first becoming obsessed with personal hygiene.

Before long, they are apt to don a white toga-like robe – hotel bedsheets are popular – before setting off on foot for the Old City or the Mount of Olives, where they will typically begin to preach a sermon, usually a plea for a return to a simpler, more spiritual life.

"They think they are Jesus themselves or they are someone who predicts Jesus's return," says Sigal Manor, an Israeli businesswoman who conducts organized tours designed around the phenomenon. "Every two or three months, I see something weird."

The pure form of the syndrome seems to affect Protestants almost exclusively, especially those from highly religious backgrounds.

Some Israeli psychologists have suggested these individuals – representing fewer than 5 per cent of the total number affected by the syndrome – have no previous history of mental instability, but this view is disputed.

In any case, the pure form is only one manifestation of the affliction.

Israeli researchers have isolated three distinct categories of the syndrome, with numerous subgroupings.

Overall, about two-thirds of those affected by some version of the syndrome are Jewish, and the rest Christian.

For some reason, Muslims do not seem to be affected, even though Jerusalem is holy to their faith as well…