It started with a niggling pain in his back. Simon, a 38-year-old advertising executive, thought he'd twisted it playing tennis, but when it didn't go away he went to see his GP. Two weeks later he was diagnosed with cancer of the spine. This story is made up, but most of us will know one like it. Someone youngish, fit and well whose seemingly trivial health problem turns out to be a life-threatening disease. Someone, as we get older, who is a relative or friend. Suddenly that cough we've had for a week seems more sinister, that headache worse than before. How can we know whether to take a symptom seriously or not? Should we go to the GP or shrug it off?

"There are many people who feel they need to go to the doctor just in case," says Dr Cosmo Hallstrom, a consultant psychiatrist in a private practice in London. "Up to 50% of people visiting outpatient clinics are having needless investigations."

These investigations are ordered by doctors also worried that they might miss the early signs of serious diseases. The health service encourages us to catch the signs of cancer and heart disease early. Screening programmes test us for diseases before we even have symptoms. Is it possible that we are getting too worried about our health?

"Our general level of anxiety and concern is on the increase," says Dr Adrian Edwards, a reader in primary care at the University of Swansea and a researcher in how people understand risk. "As we get older we have an increased realisation of our vulnerability. When we have examples close to home of friends or relatives who get ill or die, they have a high impact. People's responses can be different, depending on their personality type. Psychologists use the terms "monitors" and "blunters". Monitors become terribly interested in all the details of the illness; they get anxious and try to reduce their own risk of getting ill, almost paying excessive attention to their health. Blunters block it out and pretend nothing like that will happen to them. If you worry too much, you should worry less. If you're a blunter, you may worry too little."

Angela Jones is a 36-year-old secretary in London who knows she worries too much about her health. "I worry that things I can't see inside my body are happening to me," she says. "I worry about getting breast cancer and that my arteries are furring up. I've heard there's a scan that can see inside your arteries and I'd like to have one. I try to do things like eating salads, going to the gym, things that are within my control, to stop myself getting sick. I'm an anxious person anyway but I get fixated by specific conditions. My friend's mother died of breast cancer when I was 18. Life suddenly seems much shorter when you get over 30. You used to think you had an endless amount of time being well before.

"Also, when you see other people's loss, it makes you aware of how awful it is - how sad it makes you. You worry about something happening to you and other people being upset by it. Lots of my women friends get more anxious about their healthafter having children because they worry about how their children would cope without them."

Dr James Hawkins, a psychiatrist at the Good Medicine charity in Edinburgh, believes that it's a combination of genetic factors and personal life experiences that determine how anxious people get about their health. "If your early childhood experience was that every time you got ill your parents rushed you off to the doctors, you are more likely to be anxious about your health. If you know someone who was reassured by their doctor and told everything was OK and they turned out to have cancer, you are unlikely to be easily reassured yourself. You may think it's another example of doctors giving wrong reassurances. There is a tendency for women to be more anxious about their health but I have seen both men and women contorted by health anxiety.

"It can be difficult for people to work out what is reasonable to worry about with their health. When other health professionals ask me when they should take symptoms seriously, I tell them to ask themselves if they would still refer the patient if the patient had the same symptoms but their anxiety was taken away," he says. "All of us at times worry about funny things in our body going wrong - we'd be brain-dead if we didn't. But if you worry about your health all the time, you need to ask yourself if you have a track record for worrying about things that usually amount to nothing. If you go to see the GP quite often, maybe you are more consumed by your health than other people. Maybe you should ask yourself if you really need to see your GP when you start worrying about a health problem. But it is hard to be clear-cut on what you should and shouldn't worry about."

Reassuring people who are worried about their symptoms is often not helpful, says Hawkins. "It's certainly no good saying it's all in your head. People crave understanding; they want an explanation that fits the facts."

Dr David Tovey, a former GP who is now deputy editor of Clinical Evidence, a digest of research evidence, believes that reassurance almost never reassures. "The worst thing is if you reassure someone about a condition they haven't mentioned," he says. "Then they may worry about that as well. You have to take patients' health fears very seriously. You have to find out the reasons why they think they have something; maybe their neighbour had it, or they read about it in a book, and offer a plausible alternative."

For most of us, that niggle in the back is likely to be just that - something muscular that goes away of its own accord. Life expectancy in the UK is 80 years for women and 76 years for men. These figures are for babies born now, rather than adults born four decades ago, but most of us will collect our pensions. Edwards says that three out of four people live to at least 65. One of the problems that people have, he says, is understanding their risk of getting ill, especially from rare diseases. "People got very worried about Sars, but many more people die from diarrhoea around the world," he says. "People's attitude to risk is determined by how common the risk is, whether it is controllable, and how dreadful it is. People see cancer as much more dreadful than heart disease. It's natural to feel vulnerable and to feel the implications for other people if something happened to you, but we need to keep the size of risks in proportion."

There is no clear set of symptoms that people should worry about any more than there are guaranteed signs of being well. "People can do things to help themselves like stopping smoking, eating fruit and vegetables, drinking less alcohol and exercising," says Hawkins. "These do reduce the risk of cancer."

But the main thing people need to do is to get over their health anxieties and start enjoying life. "Some people worry about dying in our 30s," says Hallstrom. "Others banish it from their minds. Then, suddenly, when something happens to someone you know, you are forced to contemplate your own mortality and it comes as a shock to you. Then for normal people mental mechanisms kick in so they don't think about it all the time. The healthy way of dealing with it is to realise you have no choice and to make the best of what you have. Live the life you enjoy living.

"Avoid doctors if you possibly can unless you have to. Don't be preoccupied with your health and avoid the sick role even when you are sick. Some people go to work with a cold, other take two weeks off. It's not the illness you have, but the impact it has on you."

And if you can't stop worrying about the illnesses you could get as you grow older, you can do something that a colleague of mine once did. He pinned up a list of all the diseases he was now too old to get.

If you think you might be worrying about your health more than you should, check how many of the following statements apply to you:

· Hearing about Tony Blair's heart problem had you reaching for your own pulse

· You repeatedly become obsessed about some aspect of your health - a funny feeling in your leg, for example, or a twitch in your eye that gets no worse and eventually goes away

· You are visiting your GP often, and finding that he or she is much less concerned than you are about your symptoms

· Whenever you find out that a neighbour has a life-threatening illness, you are strangely keen to find out all the grisly details

· Your bathroom cabinet is filled to overflowing with all sorts of creams, pastes and pills, most of which you never use

· Your Family Health Encyclopaedia is the best-thumbed book on your bedside table.

· You're reading the New England Journal of Medicine rather than watching Green Wing

· When talking with friends, you find the conversation invariably turns to the subject of you and your health

· You always think you've got the condition that is being written about in your newspaper's health pages