Dr Brigitte Arend has some of the healthiest patients in the world: the woman who doesn't have multiple sclerosis and the man who isn't suffering from a brain tumour. But her patients don't believe it because they are hypochondriacs.
For most people, a hypochondriac is someone who thinks or imagines they are suffering from one of any number of illnesses, all except one: hypochondria itself.
As film director Woody Allen once said: "I'm not a hypochondriac, I'm an alarmist." For eight years, Dr Arend has worked at the Bad Bramstedt clinic near Hamburg that specialises in treatment for hypochondriacs, people for whom a twinge in the chest is always a heart attack and indigestion is always stomach cancer.
"Hypochondria, or excessive health anxiety, is an illness with a bad reputation," says Dr Arend. It is as common in men as in women and can be found in all age groups and social classes but is rarely "diagnosed positively", she says.
The guiding principle at the clinic is to treat the patients seriously as sick people even though they are physically healthy, with the aim of breaking the chain of anxiety that grips hypochondriacs.
"They seek constant reassurance that they are not sick and go back to the doctor more and more often, sometimes daily or even hourly," says Dr Arend. A visit to the doctor can ease a hypochondriac's anxiety, but the fear of illness grips them again as soon as they leave the surgery.
Dr Arend never confronts her patients with the fact that they are physically healthy. "Tell a hypochondriac that their sickness is psychological, and they simply feel misunderstood," she says. Instead she examines the patient for the disease they believe they are suffering from. When that has been ruled out, she works with the patient to find other reasons for their pain.
In eight years, she has seen every type of hypochondria. One of the most extreme cases involved a man who had undergone an unnecessary triple heart bypass and two unnecessary hip replacements, all performed by surgeons who simply felt sorry for him.
Another patient, an insurance salesman, checked into the clinic saying he suffered from heart palpitations and high blood pressure. Although he was healthy, he was convinced he would drop dead from a heart attack if he even climbed a flight of stairs.
During his stay at the clinic he was encouraged to participate in sport, under medical supervision, to prove that exerting himself would not mean certain death.
When Christina Nickel awoke from an operation to repair a slipped disc in her back, she didn't recognise herself. The operation went without a hitch, but the 37-year old was convinced the entire left side of her face was frozen. What's more, she could feel her limbs twitching and had a permanent ringing in her ears.
Convinced her doctor and even her husband were lying to her about the true state of her health, she visited five more doctors. She was convinced she was suffering from multiple sclerosis, even though each successive doctor gave her a clean bill of health and told her that everything she thought she was feeling was harmless and would soon go away.
"But it is absolutely not harmless, it doesn't go away," says Dr Arend. Whoever suffers from hypochondria is not a faker, she says, but someone who needs help.
After two months in the clinic, Nickel realised that the problem lay within herself.
"I wanted to be the perfect wife, mother, daughter, and for years I was so stressed that I no longer noticed that I was overexerting myself," she says.
Some hypochondria specialists believe the root cause of the illness is simply modern life.
"In a society where you have to fight for food to survive, where you have to fight for your basic needs, there aren't many hypochondriacs. When all your needs are taken care of, you have more time to worry about your health," says Dr Ingvard Wilhelmsen, a Norwegian hypochondria specialist.
There are no official statistics and few studies of hypochondria, something that only encourages ignorance, according to Prof Detlev Nutzinger, director of the Bad Bramstedt clinic.
He estimates that between 3 and 5 per cent of all patients seen by doctors are hypochondriacs, while a study in the US in the 1980s put the figure at 10 per cent.
Translated to Germany, that would mean hypochondria costs the German health system DM 50 billion (£20 billion) a year.
Prof Nutzinger hopes to get funding to do further research on the causes of hypochondria, but until then he is happy to tell hypochondriac jokes.
Derry Clarke, chef, owner of l'╔crivain restaurant, Dublin Have you heard the one about the hypochondriac who died after living to the ripe old age? His gravestone read: "Do you believe me now?"