With the exception of the extremely cautious (those who not only look both ways before they cross, but keep the Green Cross Code by their bedside), we have probably all had days when we've shuffled dreamily across the road, only to realise that the bus that seemed to be dawdling towards us is, in fact, hurtling downhill. In this case, of course, most of us rush to the opposite pavement.
But now imagine that you were unable to jump out of the way. Imagine being stuck in the centre of the road, heart pounding in your chest, stomach colonising your throat, skin breaking into a sudden, profuse sweat. Imagine being suspended indefinitely in that moment, the bus always hurtling towards you: looming, horrifying, imminent.
This is how Sarah Purser, 41, describes the extreme terror she's experienced over the past year, since her severe health anxiety (otherwise known as "hypochondriasis") reached its height. Sarah has suffered from general anxiety for almost two decades now, but has been particularly anxious about her health since the birth of her first child, Benjamin, five years ago.
"I had a horrific pregnancy," she says, "and I think that that blew all my expectations that a doctor would be able to help me if something was wrong. Because of mistakes that were made, I had my son three months early and he only weighed about two pounds. I had to stay in special care with him for a good few months.
"Between Benjamin and my second child, Lily, I also had two miscarriages. When I was pregnant with Lily, I analysed every twinge: 'Is this it? Am I going to have her early? Is she going to be alive?' I became completely aware of every little pain."
As her anxiety developed, Sarah became convinced that the smallest spasm was the sign of "a brain tumour, heart attack, cancer ... I'd be walking down the street and there would be a twinge in my leg and I'd think, 'Oh, it's a blood clot.'". She sought constant reassurance from her husband and GP, but they could never quell her fears. (This is typical. Health anxiety sufferers look for reassurance from their partner, GP, consultants, NHS Direct, internet forums, but can never be truly comforted.) One night, Sarah ended up in A&E at 3am, sure that her chest pain signalled a heart attack. Tests showed that she was fine.
Sarah is one of the subjects of the Channel 4 documentary, Hypochondriacs: I Told You I Was Ill. Also featured is Laura, who has been terrified she might have cancer since her father's death from the disease seven years ago. And Kevin, who has a morbid fear that he has either contracted HIV/Aids from a blood transfusion, or that he will fall ill with it in the future. Three negative HIV tests can't convince him that he's safe. All of them undergo a long weekend of intensive Cognitive Behavioural Therapy (CBT) in a bid to relieve their problems.
Sarah, Laura and Kevin are far from alone in their fears. As many as one in 10 people experience health anxiety, and for 3% of us it becomes a major problem. An estimated one in four doctor's appointments are the result of health anxiety, with one study suggesting that such patients use up as much as 10 times the healthcare resources of an average patient.
Over the years, hypochondria has been viewed with humour or scorn (in 1914, a manual called The Modern Family Doctor described hypochondriacs as "childlike, egoistic and self-centred"). Sufferers believe this is based on the false notion that they enjoy the thought of being ill. In fact, they argue, their anxiety arises from the opposite - a horror of illness, which makes them seek early diagnosis. Hence their preference for the more recent term "health anxiety".
Scorn certainly distracts from the genuine repercussions of this disorder: problems at work, relationship breakdown, depression. There is no doubt, too, that many sufferers experience genuine physical symptoms. Breathlessness, headaches and nausea, among many other things, can all be induced by health anxiety.
"I don't like the expression hypochondriasis," says Dr Florian Ruths, a psychiatrist and cognitive therapist who features in the film, "because it suggests that people are imagining their symptoms. What people, including GPs, have to understand, is that patients do experience real symptoms, real pain." And this isn't only psychosomatic. Many sufferers prod their body repeatedly, checking for lumps or swelling, which can, again, induce genuine physical pain.
Given the extreme emotional, and, indeed, financial costs of health anxiety, it makes sense to find an effective cure. Ruths believes that CBT is one of the best ways to treat the condition, and there is evidence to support this - a major trial at the Brigham and Women's Hospital in Boston in 2004 found that CBT can significantly improve symptoms. Before the programme, though, Ruths had never treated sufferers in such an intense setting (with four or five therapy sessions carried out in quick succession) and so wasn't entirely sure how successful it would be.
Initially, some of the behavioural experiments he uses seem more than a little surprising. For Kevin's first treatment, for instance, he is taken to a public convenience (where Ruths has smeared fake blood around the toilet and sink) and encouraged to wipe his hands around the toilet. Amazingly, he does it. Sarah's treatments include her being asked to hold her breath (she's scared that she'll pass out) and to run on a treadmill, raising her heart rate above 120bpm (she's terrified that she will collapse). In both instances she is fine.
Hypochondriacs "often misinterpret physical symptoms," notes Ruths, "but by inducing those physical symptoms we can show that they're benign. If you can induce a hyperventilation, for example, the patient realises, 'If I can induce this, it can't be dangerous, because I've created it.' We also make them anxious, so that we can explain to them, 'This is what happens when you're anxious - your heart races, you sweat, you breathe more heavily.' That is normal."
By the end of the long weekend, Kevin has stopped obsessing about HIV, and Laura is on the way to accepting her father's death. All of which holds out hope of a quick, effective treatment for those whose lives are spent in terror of their own mortality. It's important to treat this, notes Ruths, "because the fear that you're going to die is the maximum anxiety that a human being can experience. Anyone who has ever sat on a plane that is going through severe turbulence knows how that feels. We need to acknowledge the fear that people are suffering - this is definitely not self-indulgence. It is disabling."
For Sarah, that fear of imminent death - the speeding bus - has stopped. "Before, when I've had some therapy," she says, "the thought of leaving my therapist has been awful. In this case, though, I felt like I'd been given some tools and I was raring to use them in the real world. And they have worked. Health anxiety is something that I'll continue to live with, but it's much more manageable now. Before, I never had a whole day when I was fine, when I wasn't constantly thinking that I was about to die. Now I have whole weeks".
· Hypochondriacs: I Told You I Was Ill (part of the Only Human series) is on Channel 4 on February 19 at 9pm.
