Immune to assurances otherwise, they vex doctors, rack up health-care costs and exasperate those closest to them with their incessant aches and pains.
In fact, they do have a medical condition - just not the ones they think they have.
Doctors used to call them hypochondriacs, but the term has taken on negative connotations of whiners and malingerers.
"No smart doctor says to a patient, `I think you are a hypochondriac'," says Dr Wayne Katon, a professor of psychiatry and behavioural sciences at the University of Washington School of Medicine.
"That's a quick way to lose patients."
Now such patients - about 4% to 9% of people who visit a doctor's office each year - are seen as having something akin to an anxiety disorder.
In the way that some people are driven to distraction by fear of planes or spiders or crowded spaces, those with health anxiety - the favoured label - interpret every physical symptom as a sign of impending doom.
"A headache is never just a headache," says Dr Greg Simon, a psychiatrist at Group Health Co-operative.
A stomach twinge can only mean bowel cancer.
A muscle pain portends Lou Gehrig's disease, or perhaps dengue fever.
Treating these patients has long been a struggle.
While the fellow fearing dengue fever may think a blood test is in order, the doctor concludes he needs his head examined.
But reframing the disorder as one of undue anxiety rather than imagined symptoms has opened up new treatment strategies that can help the worried get on with their lives.
The average person experiences two to three inexplicable twinges, pains or aches or other odd symptoms every day, says Dr John Wynn, a psychiatrist at Swedish Medical Centre.
Most of us don't think much about them and they go away.
But for Melissa Woyechowsky, no bump or twitch went without notice.
Her glands felt swollen, so she immediately concluded she had HIV, even though she tested negative repeatedly.
Any bump on her skin was thought to be skin cancer.
Her feet felt numb and tingly, so she went online to search for an explanation.
She became convinced she had a neurological condition.
The doctor's clean bill of health didn't reassure her for long.
"On the way home I'd start to thinking, they must have missed something," she says.
Newly married and in her 20s, she became a full-time worrier who rarely left the house.
Her husband grew so exasperated with her it almost ended their marriage.
Finally, she discovered on the web a new condition: health anxiety.
"I realised it was the one thing for which I had every single symptom."
A psychiatrist prescribed Prozac, which she credits with muffling her blaring health fears.
Now 35, she hasn't obsessed about her health in five years.
For people prone to health anxiety, minor physical sensations or observations can kick off a vicious cycle.
They zero in on a symptom and start obsessing.
The more anxious they become, the more glaring the symptom.
The irony is that it is the patient's ramped-up anxiety that likely causes many of the physical symptoms, ranging from headaches to belly aches, back pain to muscle soreness to sleepless nights.
"Anxiety turns up the volume on body sensations," says Simon. "A patient of mine described the process as being like when the oil light comes on in your car and suddenly you can smell things you never smelled before."
Health anxiety often develops in young adulthood, and usually in people who have another diagnosable mental disorder - often depression, generalised anxiety disorder or obsessive-compulsive disorder.
No-one really knows what triggers it, but some cases seem to be related to a traumatic health-related event, such as a loved one dying suddenly.
Some experts speculate it might be tied to a low threshold for physical pain.
"At its essence, anxiety is the endless search for certainty you can't find," Simon says.
"And with health fears, it's true, you can never be certain they won't actually happen."
This quest can become an obsession.
"It certainly does keep me from doing the things I want to do, in that it makes it very difficult to think about a positive future.
"If I'm going to be a bedridden invalid with dementia, why the heck am I doing X or Y," says James Bayliss, a member of Health Anxiety Support.
Hypochondriacs and even garden-variety worrywarts fighting to suppress their health-related fears have their work cut out for them.
Vague pharmaceutical commercials provide rich fodder for worries: Is your good cholesterol too low or bad cholesterol too high? Could your heartburn really be gastro esophageal reflux disease? Do you "gotta go, gotta go, gotta go right now?"Websites have popped up for every disease imaginable, and 80% of internet users report searching for health information online.
It's enough to send even the healthiest and most level-headed among us scurrying to the doctor's office.
Health-information overload doesn't create hypochondriacs, but it can give them more things to worry about.
"For some, it can be taken as, here are all the new ways you can die," says Craig Sawchuk, a clinical psychologist at the University of Washington.
For this reason, a temporary form of hypochondria, called medical-student syndrome, is considered a standard phase of doctor training.
"They're under high levels of stress and sleep deprivation and they're immersed in reading about symptoms and pathology and they say, "Oh God, I'm feeling just that way," Sawchuk says.
Shannon Simon, a second-year medical student at University of Washington, says she made rather outlandish interpretations of her own bodily symptoms.
The healthy, active 23-year-old took her blood pressure and found it to be a little bit high.
Her first reaction: "I automatically thought I had an adrenaline-secreting tumour or kidney damage."
When she woke up with a sore neck: "I thought, uh-oh, that could be meningitis."
As it turns out, the elevated blood pressure wasn't surprising, considering the stress she was under, and the stiff neck was more readily explained by her lifting too-heavy weights at the gym.
So what's a time-strapped doctor to do when faced with chronic hypochondriacs?Dismissing their symptoms will only make matters worse, experts say, and will likely send them doctor-shopping.
"You always have to respect the patient's experience. You can't argue with someone who has a sensation," Wynn says.
Psychotherapy techniques and antidepressants used to treat other anxiety disorders can help quiet patients' health fears about their sensations - that is if you can get them to agree to therapy, rather than a biopsy.
In a landmark study in the Journal of the American Medical Association last March, Dr Arthur Barsky at Brigham and Women's Hospital in Boston found cognitive therapy to be surprisingly effective in helping patients dampen their overreaction to every ache and twinge.
Therapists taught patients to consider more likely explanations for their symptoms and trained them to stop habits that further fuelled their anxiety, such as continually taking their pulse or researching their symptoms online.
Of patients who attended the study's six therapy sessions, half paid significantly less attention to their symptoms and were better able to perform daily activities without fretting about their health.
The down-side was that a quarter of patients quit the study because the treatment didn't fit with their interpretation that their symptoms were "real".
Still, the results were considered a major success among psychiatrists because hypochondria is considered just that difficult to treat.
Other studies have also found some antidepressants may help patients keep chronic, overwhelming health fears in check.
"Treatment is still very limited by the fact that a lot of hypochondriacs wouldn't be caught dead going to a psychiatrist," Katon says.
"They say, `Gee, my belly hurts or my back hurts', and they are mystified or insulted when a doctor begins asking about stress in their life."
Sometimes, he says, when the one disease a patient doesn't think he has is health anxiety, all a doctor can do is run interference to protect the patient from unnecessary medications, tests, or even surgery, and to take their symptoms as seriously as anyone else's.
After all, hypochondriacs get sick, too.
Hypochondria
FOURTH TO FIFTH CENTURY BC: Hippocrates wrote of hypochondrium, referring to disorders of the anatomical region below (hypo) the cartilage of the ribs (chondros). In women, it's diagnosed as hysteria, caused by disturbances in the uterus.
SECOND CENTURY: Another ancient Greek physician, Galen, described it as a digestive disorder associated with melancholy and fear of illness.
14TH-16TH CENTURIES: During the Renaissance, hypochondria became associated with upper-crust, intellectual and creative types.
17TH CENTURY: French playwright Moliere mocked hypochondria in his classic comedy The Imaginary Invalid.
19TH CENTURY: Hypochondria became equated with melancholia, the Victorian take on depression.
20TH CENTURY: Freud's disinterest and germ theory drove the diagnosis out of favour until it was added to the Diagnostic and Statistical Manual of Mental Disorders in the 1970s.
TODAY: The American Psychiatric Association defines "hypochondriasis" as preoccupation with fears of having a serious disease based on a misinterpretation of bodily symptom. To fit the diagnostic criteria, the belief must last at least six months despite reassurances from doctors. Most psychiatrists have adopted the term "health anxiety."
Source: Hypochondria: Woeful Imaginings, Dr Susan Baur
SELF EXAMINATION
If you fret a lot about your health, ask yourself:
HAVE YOU GONE THROUGH stressful situations recently - a move, job loss or death in the family?
DO YOU HAVE SYMPTOMS of depression or anxiety, such as trouble sleeping, headaches, fatigue or stomach aches?
DO YOU HAVE ANXIETY ATTACKS - spells where you feel shaky, short of breath and your heart races?
DO YOU ALWAYS WORRY about the same thing, or does it constantly change? What's your biggest concern?
Share this information with your health-care providers so they can better address your symptoms.