A Christian tourist from Northern Ireland is missing in the Israeli desert, having left a trail of pages from the Bible behind him, with search teams believing that he may have been suffering from delusions known as 'Jerusalem syndrome'.

So, what is Jerusalem syndrome?

Let's face it: Jerusalem, with its political place as the divided city at the heart of the world's most intractable conflict, and its role as the Holy City for the three monotheistic religions, can be a maddening place to stay, let alone live.

But sadly, for some its maddening nature is literal, as those with either no history of mental illness get swept up delusionally by the atmosphere of the ancient souks, and those with a history of psychological problems have them exacerbated by their trip, with some troubled people making it their mission to visit in the first place.

Those suffering from psychosis are often misunderstood as dangerous to others, but it should be emphasised that the vast majority of sufferers of Jerusalem syndrome are harmless, though they experience some very strange ideas.

Sufferers tend to come to believe they are prophets or other figures from the Bible. I myself have seen a man dressed in white with a beard and long hair, a makeshift 'robe' that appeared to be made from a bed-sheet and a large Bible attend multiple services across denominations at the Church of the Holy Sepulchre before dawn, who local experts clearly believed was suffering from Jerusalem syndrome. He did no one any harm, but he probably ended up in the city's asylum.

There are one or two anecdotal and unverified reports that patients at the Kfar Shaul Mental Health Centre on the outskirts of Jerusalem are occasionally treated with some harshness, and that they are frequently thrown out of the country after attending. But this may be untrue. 

What is known is that psychiatrists describe the manifestation of the delusions as highly theatrical and public. Sufferers will indeed often rip hotel bed sheets into makeshift togas, deliver impromptu sermons in front of holy sites and go wailing through the souks.

'Their appearance is very dramatic and they use Jerusalem as a stage and deliberately go there to play out their act – an act that they entirely believe to be true,' said Dr Moshe Kalian, the former district psychiatrist of Jerusalem and a leading authority on the syndrome, reported in the Telegraph.

Past stories, recounted by the Telegraph, include an Irish schoolteacher who came to a Jerusalem hospital convinced she was about to give birth to the baby Jesus when in fact she was not even pregnant; a Canadian tourist who believed he was the biblical strongman Sampson and tried to tear stone blocks out of the Wailing Wall; and an Austrian man who got upset in his hotel kitchen when staff refused to prepare the Last Supper for him.

The syndrome, which has afflicted mainly Christians and some Jews but no Muslims to date, intriguingly, apparently goes back a long way. Cases have been recorded since the Middle Ages, and Simon Sebag Montefiore refers throughout his well-received 'biography' of Jerusalem to the syndrome, 'a madness of anticipation, disappointment and delusion' that over the centuries has affected kings and emperors as well as humble pilgrims.

As J E Hanauer, a British traveller and Anglican vicar, wrote in around 1870: 'It is an odd fact that many Americans who arrive at Jerusalem are either lunatics or lose their mind thereafter.'

The majority of the hundred or so who are hospitalised each year reportedly have suffered mental health problems in their own countries and came to Jerusalem on what they saw as a mission from God.

However, in a 2000 article in the British Journal of Psychiatry, academics claimed to have identified and described a specific syndrome which emerges in tourists with no previous psychiatric history.

This is the most contentious point of debate among scholars of Jerusalem syndrome, what one group of doctors has called Type III cases: people with no history of mental illness who become overwhelmed by the city's religiosity and – temporarily – lose their minds.

'The third type of Jerusalem syndrome is perhaps the most fascinating,' wrote psychiatrists from Kfar Shaul Mental Health Centre. They recorded 42 cases of people who arrived in Jerusalem as ordinary tourists, then suffered severe psychotic episodes while there, and later recovered totally after leaving the Holy Land. Tellingly, of the 42 individuals, 40 were from what doctors described as 'ultra-religious' Protestant families.

Type I Jerusalem syndrome is imposed on a previous psychotic illness, and Type II is known to be superimposed on and complicated by idiosyncratic ideas. Of this version of the 'illness', Wikipedia, always to be taken with a pinch of salt, states: 'This does not necessarily take the form of mental illness and may simply be a culturally anomalous obsession with the significance of Jerusalem, either as an individual, or as part of a small religious group with idiosyncratic spiritual beliefs.'

Which, finally, brings us to the important question: where does religious experience end and delusion begin? I myself have had several special moments in the Holy City, and at the Holy Sepulchre, sometimes at what others would regard as strange times of the day, like the early hours of the morning, with just a few nuns around. Madness? Who knows? But what is true is that Jerusalem is a beautiful, wondrous and sometimes literally maddening place.