For a while now, a spot on my chest has been bothering me. It's on the right-hand side, third rib down, north-north-east of the nipple.

I say 'spot'. It used to be a spot. That's how it started - as a no-nonsense, common-or-garden whitehead. I remember it appeared suddenly, unexpectedly, without the premonitory subcutaneous lump you usually get with whiteheads. It was very swollen, very tender, more so than most whiteheads, and it hung around for weeks without showing the slightest interest in becoming less swollen or tender, or indeed in 'ripening' (my mother's phrase) into one of the Pompeiian pus volcanoes of my adolescence.

I asked my wife: 'What do you think I should do?'

'What, generally? In life?'

'About my spot.'

She made a face. 'Leave it. You should always leave spots.'

'But it really hurts. If I put pressure on it, I get this pain radiating across my chest.'

'Then don't put pressure on it.' She turned away, back to her newspaper. 'Honestly.'

I had to do something. As I said, the spot was painful. It looked ugly, too. I'd stopped wearing shirts to work because it was visible, in the sense that an elephant is visible, with the two top buttons undone, the way I always wear shirts. I'd started wearing T-shirts every day. But all the T-shirts I owned were fitted, which is to say constricting. I wasn't going to start wearing tent-like, Christian-summer-camp T-shirts because of some stupid spot, but there was a danger with these constricting/fitted T-shirts that excessive pressure would be exerted across my ribs when I stretched or reached up to grab a hand-strap on the tube or something, and the thin film of skin over the spot would rupture and ...

Enough.

I filled a bowl with freshly boiled water into which I drizzled 5ml of TCP. I took a clean flannel from the laundry cupboard. I took the smallest needle I could find and sterilised it in a pan of boiling water for five minutes.

The spot was now approximately half a centimetre in diameter, with a yolky eye in the middle. The yolky eye was the locus of infection - Pus Central. Gingerly, I laid the hot flannel on it for five minutes to soften it up. Then I dug in, thinking vague thoughts about bayonets and the fake 'putrefaction' smell they pump out while you're wandering around the Trench Experience at the Imperial War Museum.

There was a brief starburst of pure pain; then all the goo came pouring out, both less and more than I expected. (I caught most of it in the flannel.) I washed the wound and dabbed it with the flannel until it had stopped weeping. I hadn't anticipated sudden deflation. At the same time, I hadn't anticipated the swelling to double in size so that it resembled half a boiled egg implanted in my chest. Life is full of surprises.

I squirted half a tube of Savlon over the mound. I covered it in a thick wad of gauze, stuck down with proper, no-expense-spared surgical tape. And that, I thought, was that.

I am 33 years old. Height: just under 6ft. Weight:10st 7lb. Waist size: 32 on a good day. I have never been seriously ill or had an operation that required a general anaesthetic. I'm not allergic to anything I know of, apart from magical realist novels, films starring Kate Beckinsale, and group holidays in rented villas.

Increasingly, though, I worry. I worry about illness and death.

I was born two months premature, an identical twin. My brother, Richard, died of respiratory failure after three days. When my parents brought me home from the hospital, my bones were still so soft that my mother had to keep turning me over in my Moses basket so that my face didn't slip out of shape. One night, she fell into a deep sleep and forgot. To this day, she maintains that my face is lop-sided. I maintain, pace Blake, that symmetry is fearful.

None of this augured well. But look! - here I am, against the odds.

Do I live healthily? Kind of. But this healthy living is passive rather than active. It consists of not doing things (eating junk food, smoking, drinking to excess) rather than doing them (exercising). And apparently this is not good enough, not any more, not at my age.

'You never exercise,' says my wife, who does exercise, three times a week. She hires a personal trainer, a triathlete called Helen.

Helen comes to our house in the morning, early. When she knocks I shuffle to the door in my dressing gown, coffee in hand. I call upstairs: 'Whippet woman's here!'

'Don't call her that.'

'What? Who?'

'Helen. Don't call her whippet woman.'

'Why not?'

'It isn't nice.'

I let Helen in. 'Hello,' says Helen, all smiles. 'How are you?'

Cathy skips down the stairs, dressed for action. She tells Helen, 'He's worrying about his health again.'

Helen turns towards me. 'It was your prostate, wasn't it?'

Cathy rolls her eyes. 'Last month, yeah.'

Helen, a sensible woman, has no wish to get caught up in some tedious marital micro-spat; but her urge to be polite gets the better of her. 'What's the worry now?'

Cathy answers before I can. 'He had a spot on his chest. But he popped it and it went septic. Now it won't heal.'

This is factually accurate. But it isn't the whole story. The whole story ... Well, that's a different matter. The whole story of my spot is a passable introduction to a bigger story whose erratic, tachycardiac rhythms compel more of us than we might care to admit: the story of hypochondria.

Over the past few thousand years, the word 'hypochondria' has accrued more meanings than a plague victim has weeping buboes. (Black death is on its way back, by the way.) But let's start with the most common: the contemporary meaning - the definition that springs most readily to mind when you hear the word. According to the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV, 1994), it's a preoccupation with the fear or idea of having a serious illness, based upon a misinterpretation of bodily sensations. This fear or idea must persist for at least six months despite appropriate medical examination and reassurance.

The Royal College of General Practitioners in the UK estimates that 25 per cent of all consultations with GPs relate to psychosomatic or unexplained symptoms. According to US figures, over $20bn a year is spent keeping hypochondriacs happy with unnecessary medical treatments.

At its most extreme, then, hypochondria is a medical condition in its own right, a disease in itself which needs, as the jargon has it, to be 'positively diagnosed'. This means it's not enough to rule out physical disease in a patient. The grounds for his conviction that he is ill must also be established.

Its manifestations can be extreme. Writer Carla Cantor found herself locked up in the psychiatric ward of her local hospital after she became convinced she had the autoimmune disease lupus. 'Each day during the winter of 1983, I'd catalogue my lupus-like symptoms - joint aches, rashes, itching attacks, black and blue marks, exhaustion. I'd bring the list to the doctors, begging them to confirm my diagnosis,' she writes in Phantom Illness, the book about her condition which she co-authored with psychologist Brian Fallon. On that occasion her doctors managed to convince her that there was nothing physically wrong; but the symptoms' recurrence 10 years later she saw as confirmation of her initial suspicions. 'Tests were negative, but no matter how many doctors tried to reassure me that I didn't have lupus I wasn't convinced. I knew about the bizarre and fluctuating symptoms that make lupus so tough to diagnose and that no test was entirely accurate in ruling it out.'

Eventually Cantor accepted her doctors' diagnosis: clinical depression. Fallon, whom she turned to for treatment, prescribed Prozac - this was the early Nineties, and he was researching the effects of those then new 'wonder drugs', selective serotonin reuptake inhibitors (SSRIs), on hypochondria. Fallon scored a success, and within months Cantor's life was handed back to her, mended.

The spot on my chest is now a shiny red weal, the skin around it flaky and itchy and eczematous. I've started putting hydrocortisone cream on it, but I have to be careful not to get any on the weal itself. Hydrocortisone cream thins the skin over time, and the skin covering the weal is already like gossamer. When I towel myself after a bath, it comes off. When my shirt rubs against it, it comes off. The result is profuse bleeding which can take up to 15 minutes to stop. Sometimes the only way I can make it stop is to hold a pack of frozen peas against it. Afterwards I cover it with a plaster, which works fine in the short term, but often I'll nick the skin in the process of removing the plaster and the bleeding will start all over again.

This goes on for several weeks. I would like it to be known that only after it had been going on for several weeks did I decide to visit my GP.

I tell Cathy: 'I'm going to Gimpface tomorrow.'

'About your spot?' She sounds only mildly incredulous.

'Yeah.'

'Well,' she says, 'if it makes you feel better.'

Gimpface is our name for our GP. I cannot, in all honesty, pretend that this is a friendly nickname. On the contrary, it is a nickname conceived in fury after he misdiagnosed Cathy's shingles as ringworm a couple of years back.

Obviously, my main concern is that the spot on my chest is cancerous. That's why I'm so keen for the doctor to see it. We're always being encouraged to alert our doctors to funny-looking, itchy moles and that sort of thing. My spot falls into the 'that sort of thing' category. It's definitely not a mole, but it did just appear suddenly out of nowhere. It bleeds and itches. And bleeding and itching is bad, isn't it?

The waiting room is packed. Opposite me sits one of the fattest women I have ever seen. Her thighs ooze out and under the narrow wooden chair like melting marshmallow. Febrile toddlers run amok; you can almost see the steam coming off them. Their mothers call after them feebly: 'John-Luke! JOHN-LUKE! Get over here - now!'

It's a funny old surgery. (We've since switched.) Doctors have a habit of leaving, and when they do it's as if either the ground has swallowed them or they never existed in the first place. When I was queuing at the reception desk, the old woman in front of me asked if she'd be seeing Dr Redmond, the doctor she liked best.

'Oh no,' said the receptionist. 'He doesn't work here any more.'

'He did two weeks ago,' said the old lady.

'Well, that was two weeks ago.'

'I know. That's what I said.'

'A lot can happen in two weeks.'

'Not that much,' said the old lady, sadly. 'Not when you get to my age.'

On the table in front of me is a pile of old magazines, much thumbed. Most of them are women's mags, their covers aglow with soap stars I don't recognise.

After what feels like seven hours but is actually one hour and 12 minutes, the receptionist calls my name.

I knock on the door and Gimpface calls, 'Come in!' But when I go in, he's on the phone. It sounds like he's on the phone to his partner.

'Really?' he's saying. 'Did he say that? And what did you say?'

I cough. Gimpface ignores me.

After about five minutes (consultations are supposed to last for 15), Gimpface puts the phone down. 'Sorry,' he says. 'I didn't mean to press the buzzer when I did.'

'Oh.'

'They sent you in too early.'

'Right.'

Gimpface has the build and demeanour of a rugby second-row - thick neck, meaty hands, thighs that rub together when he walks. His hair looks like the hair in the Grecian 2000 adverts: subtly aerated, parting like a Roman road. His mouth is fixed in a half-smile, and his eyes twinkle merrily, as if my presence here is the most tremendous joke - as if my predicament, whatever it turns out to be, is simply grist to the hulking, steam-powered mill of his ironic sensibility.

'So,' he says. 'What's the problem?'

As efficiently as I can, I explain about the spot. I try to keep the story straight - not to embellish or waffle, which doctors hate, with good reason. While I'm talking he stares at me, stroking his lips with his thumb.

When I've finished, he sighs and says, 'I suppose I should take a look at it then.'

I take my shirt off. He doesn't bother to get up. I say, 'It might be easier if you stood up.'

He says, 'I'm fine sitting here.'

He leans forward and peers squintingly at the spot. Then he sits back, as if stunned into silence.

Time passes.

'So?' I say.

'It's not cancer,' he says.

'I didn't ask you if it was.'

'To be absolutely honest,' he says, 'I don't know what it is.'

'Okay.'

'It looks a bit like a blood blister.'

'That's what I thought. But blood blisters go in a couple of weeks.'

'Whatever it is,' says Gimpface, 'it's so ... minor. If you hold on I'll look it up.'

From a small bookshelf behind his desk he plucks a fat manual. He leafs through it listlessly. 'No,' he says, 'nothing here.'

'Nothing at all?'

'No.'

'And you really think it's nothing serious?'

'As I said, I don't know what it is. We could have a bash at freezing it off if you like.'

How should doctors deal with hypochondriacs? In the September 2003 issue of Current Psychiatry, Brian Fallon and Suzanne B Feinstein offer some advice. Doctors need, they say, to set limits to their involvement with 'problem patients'. They suggest doctors tell their patients something like this: 'I will reassure you only at office visits (not by phone), the office visits will be limited to once a month, and during each visit I will reassure you no more than once.'

So what I want to know is, where's my reassurance? After all, it's pretty obvious Gimpface thinks I'm a 'problem patient'.

'So,' he says, watching me put my shirt on. 'How's the old prostate?'

'I still get up in the night. Several times.'

'Maybe you should drink less before bed.'

'I don't want you to tell me that I've got prostate cancer,' I say. 'I want you to suggest other things that might be wrong.'

But Gimpface has already pressed the buzzer.

Maybe it's just a manner thing. Maybe I shouldn't project, be so defensive, be so suspicious of my doctor's motives. Maybe I should be a nicer person.

I am talking to my friend Paul. Paul is a 41-year-old solicitor from Salford I met on the internet - on a health-anxiety forum. I've been impressed by the way he's managed not to alienate doctors with his hypochondria. I wonder how he does it.

'Well,' he says, 'for starters I don't call my doctor Gimpface.'

Paul says he's always maintained respect for doctors. 'I've never wanted to be one of those oh-no-not-you-again patients - the kind doctors talk about among themselves, rolling their eyes. Actually, that's one of the types of reassurance I ask my GP for - reassurance that I'm not annoying him.'

'Does he give it to you?'

'Most of the time.'

Paul admits his affection for his GP may be linked to the man's willingness to refer him when he became concerned about a 'change in bowel habits'. Understandably, he thought he had bowel cancer. 'I had to have what I can only call an "invasive procedure", but they were able to give me the all-clear immediately. I felt ... euphoria. It was like all my ecstasy hits rolled into one, not that I've had that many. It felt like being reborn, though without the religious connotations.

'The explanation was irritable bowel syndrome - stress-related irritable bowel syndrome. In other words, I'd been doing this to myself.'

I'm thrilled for Paul. This is great news.

Paul thinks my problem with doctors may be a case of 'classic Freudian transference'. 'Transference' is the idea that distortions in the relationship between a patient and an analyst - or in my case a patient and a GP - will affect the outcome of the treatment. 'People usually talk about transference in terms of someone falling in love with their analyst,' he says. 'But obviously that's not what's happened here.'

'No.'

'It sounds like things with Gimpface have got pretty bad. Maybe they've gone beyond transference.'

'You think?'

'It's possible.'

'Where's that, "beyond transference"?'

'Countertransference.'

'Meaning?'

Paul smiles. 'That Gimpface hates you, too.'

Like most forms of obsessive behaviour, hypochondria is about asserting control. If I pay close enough attention to my body, I will prevent it being ravaged by illness. If I catch illness early enough, there may be time to cure it. I can't leave my physical wellbeing in the hands of an imaginary God. Voltaire found it impossible to believe in the God who had allowed the Lisbon earthquake of 1755 to happen. I find it impossible to believe in a God who would allow me to get skin cancer, which may just be solipsism on my part, but hey.

What, then, is the upshot?

It is that I must - we must - be vigilant. We must stand before the mirror and inspect ourselves, every last fold and crease, for lumps and gristly bits and misshapen spots, and we must do this regularly, and we must take our findings to our GPs, because you never know.

You never know. But there's a slim chance that they might.

Doctor, how bad is it?

For doctors, one of the most frustrating things about hypochondriacs is their belief that it's possible to feel completely well all the time. This leads them, they say, to interpret any physical symptom as a sign that something is wrong.

But does it? Unless you've always suffered from some sort of chronic condition, surely it is possible to feel completely well, or at least to know what it is to feel well (based on a memory of childhood, perhaps, or the aftermath of a corrective operation). I think most people carry around with them a Platonic ideal of wellness. They know when they are well - when their bodies and minds are in a state of benign equilibrium - and when they are not.

The test which psychologists use to diagnose hypochondria is called the Whiteley Index. It comprises 14 questions, answers to which are graded from 1 to 5 in rising order of severity: 1 Not at all; 2 A little bit; 3 Moderately; 4 Quite a bit; 5 A great deal.

It's an interesting test to try out on yourself.

The Whiteley Index

1 Do you worry about your health?
2 Do you think there is something seriously wrong with your body?
3 Is it hard for you to forget about yourself and think about all sorts of other things?
4 If you feel ill and someone says you are looking better, do you become annoyed?
5 Do you find you are often aware of various things happening in your body?
6 Are you bothered by many aches and pains?
7 Are you afraid of illness?
8 Do you worry about your health more than most people?
9 Do you feel people aren't taking your illnesses seriously enough?
10 Is it hard for you to believe the doctor when he/she tells you there is nothing for you to worry about?
11 Do you often worry about the possibility that you have a serious illness?
12 If a disease is brought to your attention (through the media or someone you know), do you worry about getting it?
13 Do you find you are bothered by many different symptoms?
14 Do you often have the symptoms of a very serious disease?

What's the score?

Healthy people without health anxiety generally have a score of 21 +/-7 (14-28). Patients with hypochondria generally have a score of 44 +/-11 (32 to 55). These numbers are merely indications to help you find out if you have hypochondria.

If your score is high we suggest you talk to your doctor about it - maybe he can advise you where to find help. You might also get a high score if you are depressed, in which case your hypochon-driacal ideas might be secondary to your depression. The same is true if you have a specific or general anxiety disorder. In both instances, you might be advised to talk to your doctor.

· I Told You I was Ill by John O'Connell, £9.99, is published by Short Books