If you keep checking that the iron is off, or wash your hands till they're raw, you may be suffering from OCD, says Virginia Ironside
There cannot be anyone who has not left the house and then suddenly thought: "I wonder if I turned the gas off?" or "Did I lock the door?" Nine times out of 10 you go back and find you have done it. But did you know that your anxiety has actually been the very mildest form of Obsessive Compulsive Disorder?

This distressing condition is estimated to affect up to 3 per cent of the population in one form or another. Specialists say only half of those come forward for treatment; the other half are often completely unaware they suffer from it.

What is OCD? Obsessions are usually thoughts. At their mildest they may be things such as a fear that you have hurt someone's feelings - a thought that, try as you might, you cannot get out of your mind. It is the anxiety that when a Tube train comes into the station you might feel compelled to push someone under it - or jump under yourself. It could be a fear that you might hurl your young baby against a wall.

These are quite common obsessive thoughts that most of us can live with since they only occur from time to time. But obsessive thought at its worst could be a fear that you have run someone over in the car - you might have to retrace your route to find the body, even though you are fairly sure it could not possibly have happened.

It could be jealous preoccupation about a partner who you know, deep down, is faithful or a wretched obsession with sexual perversions - unrealistic fear that you might sexually abuse a child - that will not go away. It could be non-stop anxieties about having Aids or blasphemous thoughts that torment the sufferer's mind. Naturally, when the thoughts become really bizarre, sufferers feel they must keep them to themselves in case other people think they are mentally ill.

Compulsions are to do with obsessive behaviour rather than thoughts. At their mildest, they can involve endless checking of gas-taps, locks and electricity points before you leave the house - Ben Elton is dogged by compulsions to check that everything is safe before he leaves home. At their worst, obsessions may involve washing your hands until they are raw, compulsively wiping surfaces with disinfectant in case of germs, hair-pulling, picking at the skin, hoarding or becoming extremely anxious if ornaments or objects are not lined up symmetrically.

OCD is familiar to most of us from those moments in childhood when we indulged in "magical thinking". Perhaps we had to touch a certain number of lampposts or jump on the squares of the pavement in case the bears got us.

Dr Johnson suffered from OCD: on crossing the threshold of someone's house, he had to perform a curious checking and stepping ritual. But no one knows whether OCD is a chemical, genetic or psychological disorder, although it seems to run in families. Men and women are affected equally and it usually starts in early adulthood.

It can now be treated with either antidepressant drugs or behavioural therapy at special clinics at one of the Obsessive Disorder Units around the country. But psychotherapy can make matters worse, says Dr Ann Roberts, who specialises in OCD at St Mary's Paddington: obsessives have a tendency to over-analyse, so that therapy become an obsession in itself.

"Between 60 and 70 per cent of sufferers will get improvement from treatment, but 30 per cent, sadly, don't respond to anything," she says. "What I am concerned about is that so many people still get the wrong diagnosis initially and can be told they have schizophrenia, for instance, when in fact they don't.

"GPs still don't recognise that a lot of women who worry about vaginal infections really have OCD, while people who find punctuality impossible are often taking time checking everything before they leave for work."

Knowing what you have and knowing you are not alone can make things better. If you think you suffer from OCD in any way that hampers your life, send a large sae to the group that helps sufferers: Obsessive Action, PO Box 6097, London W2 1WZ.

'I go into a bit of a trance in doorways'

Chrissie, 32, started washing obsessively when she was 15 and has suffered ever since.

"I used to see my bedroom as my 'clean zone'," she says, "and I would wash constantly until my hands were raw and chapped because I didn't want to contaminate my room. I got very agitated when I cut my nails or cut my hair because of the bits. I had to wash them all away, every tiny bit, or I'd be extremely anxious. I'd wear dirty clothes outside my clean zone, and keep the clean clothes for inside my room. My parents would get angry before they understood, because they thought I was stupid."

Her father had a tic, which is related to Tourette's syndrome, and it appears now that there are connections between the OCD and Tourette's. In her twenties, the condition improved and Chrissie became an auxiliary nurse. But although she could deal with all the blood and mess of sickness, she still had to perform certain rituals because of her condition.

"It took me a long time to get to work, as I had to make sure everything in my room was symmetrical before I left. I took my problems to work, though, and if someone asked me to close a locker, say, I'd do it, but then I'd have to count to 100 in 10s, then check I'd locked it, then count again and check again ... and it would take me a long time."

She has now lost her job and is at home on medication. "At the moment I have to check the distances between everything, like from my foot to the doorway, so I don't touch anything. If I touch anything, I have to rub myself very hard and check if I'm bruised. I know I'm not, rationally, but I can't help checking."

On the surface, Chrissie appears perfectly normal. She has had a couple of boyfriends. "The last one had a heroin addiction, but he was very patient and tolerated my illness reasonably well. People might notice I go into a bit of a trance when I'm checking a doorway or something, but otherwise I seem OK. Inside I'm a wreck, and my thoughts are absolutely dominated by this compulsion. Sometimes I find I have to repeat sentences two or three times, which is really maddening because I know people think I'm such a fool, but actually I'm above-average intelligence."

'I started checking the dog, checking the symmetry of her features'

Maria Prior (above), 33, is medically retired. She has had symptoms of OCD since she was a child.

Maria's mother divorced her father when she was six because of his violent nature. Once, Maria smashed a china doll that seemed to her imperfect. At 16, she painted her bedroom but remembers feeling more than normal distress when the paint smeared. Otherwise, she was fine. She worked as a gallery attendant at a London museum for 12 years until she suffered physical and verbal abuse from a colleague. She was unsupported by her employers and this triggered an illness that led to her becoming retired with OCD.

"I became fixated on a fireplace with a mark on it in the bedroom where I used to live," she says. "I had to try to retouch it, pulling the paint off and repainting it all day long. I would then see little marks on everything and the marks would terrify me. There was one on my electric fire which frightened me and I would watch it, shaking. If there's one thing I particularly can't stand, it's hairmarks or brushmarks on paintwork. Ugh!

"I now can't get anything from a shop which doesn't allow me to take it back because when I get it home, if I find a mark, I have to return it. I am also terrified of central heating radiators. I feel sick at the thought of them being in my flat."

Finally, having become convinced that owning a dog would help her to overcome her OCD, Maria acquired Cherie, a Yorkshire terrier. But she became obsessive even about her pet. "I starting checking the dog, checking the symmetry of her features ... I got worse and worse, and ended up in hospital with a nervous breakdown, and they diagnosed OCD immediately."

Now she is fighting the Corporation of London, which wants to evict her from her flat because dogs are not permitted. "She's the one obsession I've learnt to accept and love.

"They want to transfer me to a different flat, but it has central heating radiators which would be impossible. I've got to the point where I've taken overdoses in desperation. My doctor says it's as important for me to have a dog as it for a blind person to have a guide dog. I'm now on Prozac but I'm still torn apart by this condition."

'At 19 he was convinced that he'd burnt a house down'

Stella's son, 24, works for a large supermarket collecting trolleys. She felt there was something different about him from the moment he was born. At school he had behaviour problems and learning difficulties.

"It was a terrible worry all the time," says Stella. "He was behind in development and didn't even talk till he was four, and when I took him to Great Ormond Street they wouldn't give him a brain scan, just said it was his co-ordination. I knew there was more to it but I couldn't put my finger on it, and I kept feeling it was my fault."

To make matters worse, he used to be very aggressive with his sister - it is a common symptom in sufferers of OCD to take out some of their frustration on their siblings, since they feel jealous of their normality. The boy's father was a perfectionist and refused to believe anything was wrong with him - and subsequently Stella has discovered there may be a link between excessive perfectionism and OCD.

It was not until he was 12 that Stella became acutely aware of his distress. He would ask: "Is this my shirt?", "Are you sure this is my shirt?", "This is my shirt, isn't it?" and she would always have to reassure him. His concentration was poor and she would often have to repeat things three or four times before they would sink in.

"Then at 19 he became convinced he'd burnt a house down. In every other way he was perfectly intelligent and he knew, rationally, he hadn't burnt down a house, but he couldn't get the thought out of his head."

His doctor told her that her son was simply trying to get her attention and OCD was not diagnosed until, desperate, she found a book about the disorder, went back to her GP and told him of her discovery.

"I've had years of frustration and if I hadn't been intelligent and persistent, I doubt that he'd be diagnosed even now," she says. "At one time he was even seeing a psychiatric counsellor who was doing him no good at all because she knew absolutely nothing about OCD.

"Until now everyone's ignored me. The majority of doctors know nothing about it. I've only recently been able to get help for him, at 24. He had a home assessment from a wonderful lady psychiatrist and is now trying Prozac as well as behavioural therapy, which means he has improved very slightly.

"They've been good to him at work, because during some trivial argument with a customer who burst into tears, he suddenly exploded, saying that with all the stress he had he could cry every day. Naturally his employers asked him what he meant and he explained. Now he's working in the warehouse, which is better for him because added stress is the last thing he needs.

"These days, when he comes in he has to do a checking routine at the door. If I call out 'Hello', he gets angry and has to start it all again. He's in a state of constant anxiety and sometimes he gets sucked into his own world, and then he says he knows there's something wrong with his brain and he's so sorry he get angry with me.

"He has a terror of anything being in front of a radiator and he mutters a lot to himself. I would give anything to see him better because if you met him socially you'd think he was just a lovely, normal, intelligent boy, with nothing odd about him at all."