Evidence is growing that infection may play a role in a host of chronic conditions including heart disease, multiple sclerosis, Alzheimer's disease, kidney stones and inflammatory bowel disease. The findings could have important implications for public health and point to new ways of protecting individuals at risk by vaccination or treatment with antibiotics or other drugs.
In heart disease, scientists believe that infection may be the missing link that can explain why the traditional risk factors of smoking, high blood pressure and high cholesterol cannot account for all variations in the incidence of the disease.
Research is pointing to the role of the bacterium Chlamydia pneumoniae, which is related to the sexually transmitted infection Chlamydia trachomatis, and affects the lungs.
Professor Jonathan Cohen, head of infectious diseases and bacteriology at the Postgraduate Medical School, Hammersmith hospital, London, said: "The evidence linking chlamydia with heart disease is intriguing. Studies have shown an association between coronary heart disease and chlamydia; the organism has been found in the plaques deposited in arteries and two studies have indicated that antibiotics may be effective against it. The findings ... lend support to the idea that micro-organisms might play a part in heart disease."
Even if true, this would not let patients off the hook of making lifestyle changes to reduce their risk. But in the case of heart disease even a modest contribution by bacteria could, if attacked, save tens of thousands of lives worldwide.
Enthusiasm for research into infectious causes of chronic disease was fired a decade ago when evidence emerged that stomach ulcers were caused not by excess acid but by the bacterium Helicobacter pylori. Ulcers were thought to be the result of a high-pressure lifestyle fuelled by an unhealthy diet and the idea that they might be caused by bacteria was greeted with disbelief. Today, long-term treatment with acid-blocking medicines such as Zantac, once the world's biggest-selling drug, has been replaced by a two-week antibiotic cocktail,which can cure the condition.
Multiple sclerosis, the neurological disease that affects 70,000 people in the UK, is known to occur in geographical clusters, suggesting an infectious agent.
Kidney stones are known to be the product of chronic infection of the urinary tract and Alzheimer's disease has also been associated with various micro-organisms over the years but less strongly than in the case of heart disease. The amyloid plaques (tangles of nerve fibres) in the brain that are characteristic of Alzheimer's may be the result of chronic infection.
Inflammatory bowel disease is known to be linked with infection by Mycobacterium paratuberculosis, a bacterium that causes Jern's disease in cattle and is related to tuberculosis in humans.
Professor Cohen said vaccines against chronic disease were a possibility but it would be years before they were a reality. "It is not inconceivable but it is not just around the corner."