Other major causes: Mastocytosis, over-use of anti-inflammatory medications such as aspirin, excessive smoking or drinking, erratic diet, family history. Whilst not thought to be a cause, stress can exacerbate the symptoms. Some chronic disorders such as emphysema, liver disease and rheumatoid arthritis may increase vulnerability to ulcers.

But for many others it is a persistent and chronic condition that affects their quality of life, keeping them awake at night, and causing regular discomfort.

Whether the condition is becoming more common is unclear, but experts say that its cost to the NHS is significant. Writing in the British Medical Journal in September 2001, Richard Logan, a professor of epidemiology, and Brendan Delaney, a reader in primary care and general practice, say that dyspepsia has "a considerable impact" on the NHS.

At any one time, they say, up to four per cent of the population are thought to be taking drugs for dyspepsia: "In the past few years the costs of these prescriptions have risen dramatically and now account for over 10 per cent of drug expenditure in primary care (471 million in 1999 in England and Wales), although this may have peaked."

The prime suspect in causing 40 to 60 per cent of chronic dyspepsia is the bacteria Helicobacter pylori (H-pylori), a highly motile (capable of spontaneous movement) bacillus which lives only in the mucus layer of the human stomach.

And although as many as 60 per cent of people with dyspepsia will have no detectable underlying cause for their condition, Logan and Delaney estimate that at least 15-20 per cent may have peptic ulcer disease (PUD).

An ulcer is an actual break in the lining of the stomach (gastric) or duodenum (duodenal), causing pain often interpreted by sufferers as indigestion, heartburn or hunger.

H-pylori is widely considered to be the principal cause of PUD, yet infection, although widespread, causes few symptoms - indeed more than 70 per cent of those carrying the bacteria are asymptomatic.

According to Robert Logan, senior lecturer in gastroenterology, and Marjorie Walker, senior lecturer in histopathology, writing in BMJ in October 2001, H-pylori is one of the "commonest bacterial pathogens in humans". Although the infection rate is falling in most developed countries, rates continue to rise in the third world, with seropositivity increasing "with age and socio-economic status".

As it is only seen in humans, it is believed that infection with the bacteria occurs mostly in childhood, possibly between parents and siblings by gastro-oral transmission - and via the gastro-faecal route in the developing world - and is considered an occupational hazard for gastroenterologists.

And H-pylori is not only implicated in PUD. Logan and Walker estimate that about 15 per cent of infected individuals will develop PUD (duodenal or gastric) or gastric cancer "as a long term consequence of infection".

Gastric, or stomach, cancer is the sixth most common cancer in the UK, with 10,000 people diagnosed every year. It is slightly more common in men than women. In eight out of 10 cases the person is over 60 years of age. Cancer Research UK says that many people initially diagnosed with stomach cancer have had symptoms of burping and indigestion for years. However many people do suffer these symptoms and never go on to develop cancer. It estimates that "about one in every 50 people going to the doctor for the first time with indigestion and burping will have stomach cancer".

However Cancer Research UK is hesitant to point the finger directly at H-pylori. "This is a bug that has been investigated a lot in the past few years. Infection with these bacteria can certainly increase the risk of a particular rare type of stomach cancer. How it is related to other commoner types is less clear," it says. And, it argues, although some researchers think it can increase the overall risk by about three times the average, other "very recent research has hinted that, although it may be linked to cancer of the lower part of the stomach, it may actually protect against cancer of the top of the stomach".

Nonetheless there is a correlation between the presence of H-pylori and stomach disease, although widespread screening for the pathogen would be a wasteful exercise as so many people are infected and yet few will go on to develop any kind of disease.

Logan and Delaney recommend that doctors use the "Alarm" symptom method in patients with dyspepsia to try to identify those who may be developing malignant disease: Anaemia; Loss of weight; Anorexia; Recent onset of progressive symptoms (less than three months); Melaena (blood in the stool) or haematemesis (vomiting blood); and Dysphagia (difficulty in swallowing).

However Logan and Delaney argue that testing positive for the bug is so highly predictive of endoscopic abnormalities that "endoscopy workload would have been reduced by 23 per cent if only the patients seropositive for H-pylori had been investigated".

And as assessment of dyspepsia and abdominal pain is one of the most common reasons for referral to hospital, with one endoscopy for every 100 adults in England and Wales carried-out in 1996, a more targeted approach would have an immediate impact on NHS costs.

However Cancer Research UK remains sceptical about the link between H-pylori and gastric cancer, a disease which is in decline. "Millions of people are infected with these bacteria and most of those do not get stomach cancer so there must be other factors at work. It may be that there are different types of helicobacter pylori and that some are less potentially harmful than others. The jury," it says, "is still out."