Some scientists believe positive thinking can reduce illness
When disease strikes, our first reaction is often to pop a pill or seek a doctor's advice.
But is it possible that having a positive outlook on life could not only reduce the risk of illness but even aid the recovery process?
The idea that the power of positive thinking has the capacity to combat ill health is one that has fascinated researchers for more than 100 years.
Countless studies have analysed the impact that a patient's frame of mind can have on their chances of surviving killer conditions such as heart disease and cancer.
Yet for all its efforts, the scientific community remains as divided as ever.
The most recent investigation, published this month, revealed cancer sufferers who remained optimistic during treatment had no more chance of surviving the ordeal than those who felt burdened by their illness.
Experts at the Peter MacCallum Cancer Centre in Melbourne, Australia, examined 170 patients and found having a dogged fighting spirit was no guarantee the disease would not result in death.
Despite the gloomy findings some experts hailed the five-year study as a welcome relief for patients who feel unable to put on a brave face when battling a killer disease.
Professor Amanda Ramirez, from the charity Cancer Research UK, said: "This study tells people it's OK to grieve after a diagnosis of cancer.
"The patients I worry about are those who have no reaction and go straight into fighting spirit mode because that's not realistic."
But the Melbourne study, published in the journal Cancer, focused on lung cancer - one of the most deadly forms of the disease, with a poor survival rate.
Other cancer research provides conflicting evidence. In the early 1990s, scientists reported on the health of women who had a mastectomy for breast cancer.
They had measured their psychological response three months after the surgery and then checked their health at five, 10 and 15 years.
What they found was that the mental attitude of the patient was a better predictor of their chances of surviving than the size of the tumour, its severity or the patient's age.
And a 1999 study showed women who felt helpless after breast cancer were more likely to die or relapse within five years of diagnosis.
If looking on the bright side of things really does keep you healthy, it may do so by influencing the release of certain hormones in the body.
So if there are fewer stress hormones, for example, the risk of heart disease may be lowered.
One Canadian study showed heart attack victims who remained upbeat about their future were three times less likely to suffer a second attack within a year.
But the exact nature of this relationship between positive thoughts and health remains a mystery.
Psychologist Oakley Ray, from the Vanderbilt University in the US, has reviewed 100 years of research and believes the secret may lie in psychoendoneuroimmunology, or PENI for short.
This is the interaction between the mind and the endocrine, nervous and immune systems.
"There are pathogens that can live in equilibrium with us - like tuberculosis - with only a small percentage developing symptoms and exhibiting illness,' said Dr Ray.
"Those who don't get sick probably have a well functioning PENI system."
According to Professor Ramirez, although optimism may not directly affect the outcome of cancer, it can be an important coping mechanism - providing the patient is not in denial about their condition.
"Having hope and being optimistic is very important from quality of life point of view, rather than a quantity of life point of view."
Professor Cary Cooper, an expert in psychology and health at Lancaster University, says a key factor may be how patients define positive thinking.
Bottling up emotions and putting on a smile is not likely to help but expressing feelings and talking to friends and relatives could.
"Will that reduce the size of a tumour? The answer is I don't know but I do know that these strategies may enable the kind of treatments these patients are having to be more successful."