"I fell in love with The Stranger's face. I looked at a man's face and into his eyes on a screen and I believed him. `If it doesn't begin it can never end.' That's what I wrote to him."
So Nicola Pagett, the former Upstairs Downstairs actress, bravely chronicles in her autobiography the beginning of her descent into obsessive manic depression as she falls in love and becomes obsessed with a man she sees on TV whom she nicknames "The Stranger".
Yesterday it was claimed that The Stranger was in fact the Prime Minister's press secretary, Alastair Campbell. As her disorder worsened, Pagett wrote hundreds of letters to him, sent him a cheque for pounds 6bn signed "Moi" and under the delusion that it was his instruction, falsely accused her husband of having an incestuous relationship with their 15-year-old daughter and feeding her heroin.
Psychologists agree that the roots of obsession are to be found in all of us. "There is a normal side to this," says Professor Petruska Clarkson, consultant psychologist and author. "When you first fall in love you can't think of anything else but the other person and what they are doing."
But for some, infatuation becomes obsession - and when the object of your desires is a famous person it is particularly potent. Helena Bonham- Carter, Madonna and Monica Seles have all had to take action to cope with obsessed fans. At the most serious level, John Hinckley shot President Reagan to impress film star Jodie Foster, and John Lennon was killed by Mark Chapman.
Little is known about delusional disorders - although delusions are common symptoms in schizophrenia and manic depression (Pagett's disorder) - people can exhibit delusional erotomania with no other symptoms: "Otherwise they lead a perfectly normal life," says Dr David Nias, a clinical psychologist at the University of London, who feels that because of a paucity of research we often have difficulty diagnosing the disorder.
But what is the appeal of the famous? "If you're going to have a fantasy lover, you're going to go for someone famous because you simply want the best," reasons Dr Nias. "Famous people have got something going for them - they're beautiful, successful and powerful, not the shabbily dressed people we see every day."
Professor Clarkson, who has specialised in the psychology of celebrity added: "Famous people represent for us our dreams, our hopes, our aspiration, our wishes, our fantasies. People project their aspirations onto the famous and make them larger than life. It's a very, very complex relationship.
"Famous people are the carriers of our projections. We aspire to be like them - we would like to be as beautiful, successful, acclaimed as they are. That's the positive side. The negative side is that people invest in the famous so much that they cannot live their own lives. They become a vicarious projection for us. And in cases like those of John Lennon and Jodie Foster it can become very destructive."
Their sheer recurrent visibility adds to the danger of becoming the focus of someone's obsession. "People on television are in our living room and thus reality can become a little bit blurred," says Professor Clarkson.
"Famous people often report being approached by people in the street and being asked what they did last week as if they knew each other. That's what we went through with Diana. Someone said: "I've seen more pictures of Diana than I'll ever see of my mother."
What makes people progress from the harmless crush to full-blown obsession and delusion is difficult to say, but Professor Clarkson believes that it is a combination of predisposition, the environment in which you grew up and life stressors you may be experiencing. On top of that, many are more vulnerable after suffering a loss: "People who have lost a child or suffered a shock or a bereavement - the delusion can serve to fill the space."
Someone who has suffered an obsession is the radio disc jockey Mike Read. He was the subject of the attentions of a woman who has now changed her name to Blue Tulip Rose Read and believes she is married to him. He says he copes with those who have pursued him by trying to distance himself from it.
"It comes with the job, but lots of jobs have far worse downsides. It doesn't really bother me, although it can be disturbing when when you're eating lunch and realise that people are watching you. Or they send photographs of your driveway and you realise they've been there and been all over your house while you were out. I think you just have to keep remembering that it's not their fault. They are not bad people."
Dr Nias thinks that more research must be done into delusional disorders so that we can begin to understand and treat them.
"For someone suffering from delusions it's like the most important love affair ever," says Dr Nias. "The preoccupation of being in love is the tragedy of it. What should be the most fantastic experience of your life ends up being the most tragic."