Few of us can imagine what it's like not to have a memory, to have such severe amnesia that we cannot remember even the moment which has just passed.

But that was the tragedy which struck Clive Wearing, a brilliant BBC conductor and music producer for Radio 3, and devastated the lives of both him and his young wife, Deborah.

Today, 20 years later, Clive, now 66, lives in the present tense. He can be speaking to you one second and not remember the conversation the next. Memories disappear with every passing moment.

If a visitor leaves the room for a minute, Clive will not remember they have been there and will greet them like a long lost friend when they re-enter.

He cannot engage in a real conversation because he becomes too confused. Ask him a question like, "How are you?" and he'll say he doesn't know, you'd better ask his wife.

The cruelty of the situation is exacerbated by the fact that every other part of his brain is functioning normally. His long-term memories are intact, so he can talk about his past life, although he can't remember what he's told you.

He still has all his other faculties too - intelligence, humour, charisma.

He remembers close relatives and his wife Deborah, now 47, who has lived with this nightmare for the best part of 20 years. They met when she was just 21 and in the choral society he was conducting.

In March 1985, he thought he was coming down with the flu, complaining of severe headaches.

But it was much more serious - the memory part of his brain was being destroyed by the herpes simplex virus, which usually only causes cold sores but, on this extremely rare occasion, crossed from his blood into his brain.

Clive has often been described as the worst case of amnesia doctors have ever seen and in 1985, Dr Jonathan Miller filmed Clive's story - Prisoner Of Consciousness - when it was discovered that the musical part of his brain had survived. He could still read music, play the piano and conduct.

'Once he's inside a piece of music, it has the impulse and the structure to carry him from bar to bar. Music is a place for him where he's safe, where he's not amnesic. When you are in a melody you know where you are. You just follow the music through. The moment the music stops he falls back into the abyss, the nothing-land,'' says Deborah.

The effect the trauma has had on her life is unimaginable. It has robbed her of the possibility of children, although she is philosophical about it.

''All I knew was that he was in this hellhole and how on earth were we going to endure it. But however hard it was for me, my suffering was absolutely nothing compared with what he suffered.

''He was extremely disturbed and distressed by his condition. When he saw me, not knowing how long ago it was since he'd seen me, he'd rush towards me and sob and sob.''

He would be worse when he saw Deborah because he was so stressed at the state he was in.

''Every time I went into the room, he would have this hiccuping, belching, fitting scene and fall back on the sofa and not be able to breathe.

''He was in an acute psychiatric ward and the domestics were too frightened to clean his room because he was so aggressive.''

Deborah herself was never scared of Clive, even though some of his violent outbursts were directed at her when she took him home at weekends.

It was only when a friend came over one day that the severity of the situation hit home.

''She came in and saw that he'd knocked me to the ground and was gripping my wrists and bellowing in my face. She feared for my safety. She drove us back to the hospital and told the sister what she'd seen.''

From that point, Clive's weekend visits home ceased.

For nearly seven years, he was kept in the psychiatric unit of the London hospital where the ambulance first dropped him off because there was nowhere else for him to go. Deborah, meanwhile, searched for treatments and campaigned for better care, setting up the Amnesiac Association.

Of course, the years of turmoil took their toll on her.

After nine years they divorced and, in an effort to move on and rebuild her own life, she went to America but returned after several disastrous relationships.

''I was never going to love anyone more than I love Clive. But I had to have space, I had to put some distance between me and the situation. I had to recover.''

She says despite the divorce, she never stopped being his wife.

''Here was a man that I absolutely adored and who absolutely adored me and we couldn't live with each other.''

After three years, she returned permanently to England and found that her feelings for Clive hadn't changed.

''I was still broken-hearted and he was still brain-injured. But what I did understand was that I couldn't stay away forever.''

Now she has written Forever Today, described as a memoir of love and amnesia.

''It's just excruciating to think about the hell hole that Clive has been trapped in, that no memory, that blinkered moment of existence that he's in. To try to imagine his situation is worse than watching a horror movie.''

Against all odds, Clive has improved in the past few years. Deborah puts it down to finding God and prayer - she and others began to pray for Clive in March 1999.

''Before, he was so upset, distressed, withdrawn and angry. He was pacing up and down. He would sit in his room and couldn't mix with other people or watch TV because he couldn't follow it.

''He became a lot more relaxed and at peace, more friendly and able to join in with the other residents in the house. He became able to listen to music and enjoy it. Before, he'd only been able to play music, not listen to it because it was too disturbing. He could sit and watch sport and even movies.

''Instead of repeating the same few conversations which he did for the first ten years, he started to say fresh things.

''In the last three to four years, he listens to what I say and remembers what I'm saying. He follows the conversation and it's much more of a real dialogue.

''Before, I'd say something dramatic like, 'Your mother died' and he'd just momentarily be upset and comment on it and immediately move on and forget all about it. Now, last year when I told him that his uncle died, we had quite a conversation about it and he immediately started talking about his uncle in the past tense and he has never talked about him in the present tense since.''

They renewed their marriage vows in 2002, partly because of her new-found faith in God, although they are not now legally married.

Deborah is now an NHS press officer and sees Clive two or three times a month. He has lived in a care home for the brain-injured in Sussex for the last 13 years.

''I frequently think, 'Why couldn't he come home?', but my heart runs away with me. Then when I take him out, I realise he's quite a responsibility. But I'm holding out for the whole miracle.''

* Forever Today, by Deborah Wearing (Doubleday, £14.99)