A woman who combs her carpets every day. A man who covers the plughole of his bath to stop his brain draining away. What links them? they both suffer from obsessive-compulsive disorder, an affliction affecting 1 person in 30
you know what it's like when you can't get a song out of your head. Imagine if you woke up one morning with Tears for Fears' "Everybody Wants to Rule the World" on your brain and it got stuck there, for a year, the soundtrack for everything you did, every minute of every day.

You know what it's like when you're suddenly irritated by something in your home which is not where it should be - the socks on the floor, the cup of congealed coffee on the windowsill. Imagine if you couldn't leave the room until you'd gone through your carpets with a comb to make sure the strands of the pile all went in the same direction.

You know what it's like when you feel a bit slow on the uptake, forgetful, brain-dead. Your body's there, but your brain is still down the pub, or in bed, or wherever. Imagine - you probably won't be able to imagine this one - that you're obsessed by the thought that your brains are spilling out of your ears. You have to put a tea-strainer in the plughole when you drain your bath so your grey matter doesn't slip down the sewer. You can't leave a room without checking you haven't left your brains on the floor, and (not being able to trust your judgement) you ask your friends to help you look, just in case.

There is a man who doesn't hate to imagine this scenario. It's the story of his life. He, along with the carpet comber and the person who literally can't get songs out of their heads, suffer from Obsessive-Compulsive Disorder (OCD), a mental illness affecting as many as one in thirty, according to a recent study in the US. In Britain, where the culture of eccentricity is not only tolerated but celebrated, symptoms of OCD - which include counting, hoarding things, cleaning rituals, doing things in a specific order a specific number of times, and having obsessive thoughts about harming others - are often dismissed as quirks. That, and the stigma attached to any mental illness, is why many in Britain suffer in silence.

Some forms of OCD have tell-tale signs, such as the chafed hands of the compulsive handwasher. By the time an OCD handwasher is on the Dettol, the skin on his hands might be so raw and sensitive he is far more likely to cause an infection than prevent one. But, then, this is a logic-defying illness. Although people with OCD are not "mad", in that they know they are behaving or thinking irrationally, they often think other people will think they are crazy, which makes them reluctant to seek help.

Obsessive Action, a self-help organisation for OCD sufferers and their families which hopes to re-educate the public about this condition, has just become a registered charity. Of course, the flip side to a public awareness campaign about any recognised or labelled syndrome is that it works too well, and we start eyeing up the friend at work who has to have everything on his desk "just so" as a suitable case for treatment.

Certainly the line between being "obsessively" tidy and the rituals of symmetry (eg straightening all the pictures) associated with some forms of OCD is often blurred, but it does not necessarily follow that very neat people will go on to develop OCD. One of the signposts of the disorder is the way the obsessive thought ties in with the compulsive action. Children do this when they avoid the cracks in the pavement so as not to break mother's back. Eventually, they realise there's no connection.

People with OCD can't make that link. They operate on the "Yes, but what if ..." principle. Desmond Kelly, medical director of the Priory Hospital, Roehampton, who has treated many OCD sufferers, explains: "If we are given a series of stimuli we get used to it and stop responding. People with OCD don't stop responding, and go on with these perpetual patterns of thought or action."

It sounds a bit like playing the lottery (using "magic" numbers to win, say) or even praying. In The Boy Who Couldn't Stop Washing, Judith Rapoport's fascinating account of her work with OCD patients, there's a list of symptoms comparing scrupulosity in the Catholic Church and OCD. She writes of a Jewish boy whose family wasn't particularly religious but none the less developed a complex and bizarre series of rituals connected with the imagined displeasure he was causing God.

Certainly, in the Orthodox Jewish community, purification rituals - including lots of handwashing - are part of the culture. David Veale, a committee member of Obsessive Action and consultant psychiatrist at Grovelands Priory Hospital, does not say if OCD occurs in greater numbers in religious communities, but when a religious person does suffer from it "their rituals go beyond what religion requires".

Sometimes the symptoms of OCD go beyond the boundaries of the condition itself, slipping into other forms of anxiety or depression. Some of the avoidance behaviour, to the untrained eye, looks like agoraphobia. "Supposing a person had his palm read in Oxford, and was told something bad would happen," Dr Veale says. "Then he could no longer go on any motorway leading to Oxford or near the clothes he wore in Oxford because they're 'contaminated'."

One man who attends one of the Obsessive Action self-help groups has such an obsession with dog faeces that when he dropped a large cheque on the pavement, where a dog may once have defecated, he could not pick it up. When he found dog mess in front of the block of flats where he lived, he had to move. Similarly, the woman who combed her carpets finally had to live in a hotel.

To the mystified or unsympathetic, it would seem the obvious cure for OCD is to keep busy. But it doesn't work like that, explains Dr Veale. "Keeping busy may distract for a short time, but it's not a long-term solution. It's a vicious circle because the OCD may interfere with work or the responsibility of work may aggravate it. For example, if you are looking after a small child and think about all the possible consequences if you get it wrong, it could get worse."

One of the main types of treatment for OCD is behaviour therapy, in which the therapist helps the patient to "unlearn" the thought or action patterns. "The role of the therapist is to do response prevention," Dr Kelly says. "We start with something that is moderately easy to overcome and go on to the next least difficult task. For example, we'd try to make an obsessively tidy person tolerate being in an untidy room. At first, all we'd do is make the pictures a bit squiffy."

Stopping thoughts is a bit trickier. We all know when someone tells you not to think of your tongue, it's all you can think about. In treating OCD, there's a process called thought-stopping. Dr Kelly explains what sounds like a fairly primitive technique. "Some people wear an elastic band on their wrist and when the obsessive thought comes into their head they flick it, which gives them a quick jab of pain."

Another technique, thought-switching, is outlined by Frank Tallis in his book Understanding Obsessions and Compulsions. This involves associating every negative thought with an unrelated positive one.

Sometimes behaviour therapy is used in conjunction with drug therapy, usually a new kind of anti-depressants called SSRIs, which control the flow of the neuro-transmitter serotonin.

Some studies in the physiology of OCD suggest a link between it Tourette's Syndrome - a condition in which sufferers curse, bark and have violent tics at random. Both are thought to stem from the same part of the brain, though the person with OCD might be obsessed with the thought that he will blaspheme in church, whereas the person with Tourette's will do it.

Although it is hard not to raise an eyebrow at some of its more bizarre manifestations, and the absurdity of OCD is not lost on sufferers, the most disturbing aspect of it is how it mimics or magnifies the commonplace. If you have to drink your coffee from your special mug, or ride the left- hand front seat of the upper deck on a bus, does that make you a creature of habit or a candidate for the analyst's couch? Is there a type of person more susceptible to OCD? Dr Kelly says: "Only a tiny percentage of people who are perfectionists will go on to develop OCD. There is sometimes a family history, so there may be a genetic predisposition. Although it's classified as a neurosis, it's one of the most severe, and at times borders on, or can be as destructive as, psychosis." This is a sentiment echoed by Dr Veale in his essay on "Friday the 13th and Obsessive-Compulsive Disorder" in the British Medical Journal: "For some patients, Obsessive- Compulsive Disorder remains a chronic handicap as severe as schizophrenia."

8 For information, write to Obsessive Action, PO Box 6097, London W2 1WZ

MICHAEL, 54, is a former hairdresser who runs a self-help group for people with OCD

"I've had OCD in excess of 30 years. It started when I was 11. I had an obsession about glass, that someone would get hurt by it. Then I had one about the number six. I used to think something would happen to my mother if I didn't perform certain rituals in certain numbers. At 15, I left school to become a hairdresser and I had a remission for eight years. At 23 it came back with a vengeance. The day it started I was driving up Crystal Palace Hill, and a motorbike with a sidecar came a little wide from the roundabout and it triggered off this thought: 'What if I had hit him?' It's worse when I'm being driven and I'm not in control. When I drive I think someone must have walked out in front of me, so I go back and check and check again. I'll go back 10 miles if I pass over a zebra crossing and don't stop to let people cross. I know full well, rationally, that nothing has happened, but I must go back and check. Now it's escalated to walking in the street. I feel my presence is going to cause an accident. At the maximum, if a person is walking very slowly then the journey from the Tube station to my house is 10 minutes - but I can take in excess of an hour. I live opposite a fire station and if they set off during the day I get a thought in my head that maybe they've knocked someone over without realising so I'll have to go out and check.

"It's drained me. I wouldn't want to use it as an excuse, but the more time you have, the worse it is. I'm not agoraphobic but I have a fantasy about being like Hugh Hefner in his house with everything in it, my own movie theatre etc. Life revolves around him so he doesn't have to go out. If I could only be somewhere without traffic. I went to Venice. I thought, great, no traffic. Then I started to think about people falling in the water."

SARA, 32, is a former auxiliary nurse.

"I've suffered from OCD for half my life. At 14 I had a lot of checking problems, counting problems, and contamination fears. If I moved from a dirty zone to a clean zone, I had to wash my hands in a very ritualistic way.

"I had remission for 10 years when I was a nurse. Unfortunately I then had a depressive illness which brought on my OCD in a different form, which was to do with checking my environment. I also had a massive panic attack which left me very agoraphobic, and the OCD habits are now linked with phobias. I feel I have to keep checking my distance from things. I do visual checks. I don't like things touching me, so I have to visually check the distance from myself to a solid object like a table or a doorway. I often get stuck and then I move on. It's quick, but it's constant. Every few minutes I'm doing some sort of visual or physical check.

"My personal goal is to get my life to maintenance level. I'm optimistic that with a mixture of medication and cognitive therapy, and possibly hypnosis, that I'll be able to lead a fairly functional-to-normal life. My parents have been very understanding. In my relationships, I invariably take on lame ducks, people with more problems than myself, so I think more about their problems than my own."