John O'Connell: Doctor, I've had another twinge...

Hypochondriacs don't want to be ill, we want to see the doctor long before anything nasty happens

Some hypochondriacs worry about heart disease or avian flu. For me, it's cancer. Not just any kind, either: a brain tumour. And not just any kind of brain tumour: a butterfly glioma, which drapes itself across the brain, a wing on each hemisphere.

As it happens, I can trace this phobia back to its source: Mrs Wilson, a teacher at my primary school who went off on holiday one day and came back with an eyepatch and a wig. A couple of months later she vanished again, on another holiday which, for all I know, she continues to enjoy. But I doubt it.

Maybe it's the internet's fault, maybe it's the media's for fanning the flames of health scares such as BSE and avian flu; but hypochondria is booming. The Royal College of Medical Practitioners in the UK estimates that 25 per cent of consultations with GPs relate to psychosomatic or unexplained symptoms. According to US figures, about $20bn (£11.5bn) is wasted every year on unnecessary treatments. At its most extreme, "persistent health anxiety" is a serious medical condition in its own right, a species of obsessive-compulsive disorder that needs to be treated with therapy and anti-depressants.

This wasn't always the view of the medical establishment. A manual from 1914, The Modern Family Doctor, attributes hypochondria to moral weakness and cowardice. Hypochondriacs, it says, "remain childlike, egoistic and self-centred, and they may be specious, plausible and good at making excuses". In these tolerant times, it's assumed that everyone is hypochondriacal to some degree. But what do these degrees look like? When does rational concern become irrational obsession?

I know, because people are always telling me, that I'm stupid to worry so much; that cancer is mostly a disease of old age whose prevalence reflects little more than how much longer we all live nowadays. But come on - cancer is scary, partly because it has a scary sounding name (from the Greek karkinos meaning "crab": I will never eat crab sticks again), and partly because it can - as the recent death from lung cancer of the non-smoking writer and TV producer Harry Thompson showed - be cruelly, bafflingly random. One in three people develops cancer during his or her life. One in four dies from it.

As a hypochondriac of long standing, I'm familiar with the misconception that hypochondriacs want to be ill. What they actually want is for any illness they may have to be diagnosed early, so that any treatment they may need has a decent chance of working. Like so much else in our lives, it's all about control. Juggling work and relationships and children is hard work. We don't want illness to take us by surprise, but to fit politely and efficiently into the schedule alongside romantic weekends away and trips to Legoland. By worrying about diseases in advance, we feel we're somehow conquering them before they've been contracted.

A doctor I spoke to said that what frustrated him most about hypochondriacs was their belief that it was possible to feel completely well all of the time. This led them, he said, to interpret any physical symptom as a sign that something was wrong.

Arguably, though, we lead more stressful lives, which perhaps accounts for the recent resurgence of the theory of the "cancer personality" - put crudely, that people who internalise stress and anger are more likely to get cancer than drivers who make rude gestures at cyclists.

This theory was debunked by the late Susan Sontag (who died of cancer) in her book Illness as Metaphor; but I always liked it. Indeed, I've started to wonder if there isn't something in it after all, possibly because the older I get, the more conscious I am of the massive amount of anger I suppress on a daily basis. It must be going somewhere and doing something.

The website Drbrodie.com lists the attributes of the cancer personality in full. And I have to say, it isn't great news. "Dutiful, responsible, hard-working" (that's me!); "exhibiting a strong tendency toward carrying other people's burdens" (oh, yes); "having a deep-seated need to make others happy" (but of course!). Another site, healthrecipes.com, focuses on anger solutions. "It is important for such people to find a channel for them to ventilate," it declares, not quite grammatically. "Maybe an aggressive sport such as tennis or maybe bowling?"

Maybe. Except that sport is one of those things I just don't do. There's nothing else for it: if I want to avoid cancer, I've got to discover my inner Russell Crowe.

Though maybe the Formula One legend Sir Jackie Stewart is a better role model. Every year he and his family fly to the Mayo Clinic in Minnesota for a full body check-up. It sounds extreme, but as a result of his vigilance not only Sir Jackie but his wife, Helen, and son, Paul, have had melanomas removed before they could do too much damage.

Some might say this is extreme behaviour that ought not to be encouraged. (One expert, Dr Ingvard Wilhelmsen, thinks that "in a society where you have to fight for your basic needs, there aren't many hypochondriacs".) But in a world where avian flu is moving steadily closer and everything we touch is a potential carcinogen, what's the harm in being cautious?