Scientists believe they have found a genetic "master-switch" that they hope to use as a way of turning off an asthmatic attack.

They have identified a group of genes that evolved to protect children against intestinal parasites but which, in a more sanitary, modern environment, cause a misguided attack on the respiratory airways. The researchers have isolated a region of DNA near the genes that appears to control their activity but which does not itself act as a gene - or "coding" region.

Richard Locksley, Professor of Medicine at the University of California, San Francisco, leads the team who will publish the study in the May issue of the Journal of Immunology. He said: "Our experiments show that all three genes are regulated by the same non-coding DNA region, and interruption of this control affects all three genes at once. By blocking the activity of this region, we should be able to block the expression of all three genes."

The three genes are responsible for the production of proteins called cytokines, which are key components of immunity and are involved in fending off disease as well as triggering asthmatic attacks.

William Cookson, a specialist in asthma genetics at Oxford University, said the cytokine genes probably evolved in the distant past to protect children from the infections they would have been exposed to during a more "natural" upbringing. He said: "Either you are good at dealing with parasites or not and, if you don't have to deal with parasites any longer, you get asthma. What these researchers are saying is that they have found a master switch for asthma. If you can learn to switch it off then that might help sufferers."

Many children in developing countries become infected with intestinal parasites, such as hookworms, which bore into the body through the soles of bare feet, from where they make their way to the gut. Such infections cause the cytokine genes to go into overdrive, producing proteins that stimulate the immune system and cause mucus production and muscle constrictions to expel the parasites from the intestine.

Unfortunately, the response becomes misdirected in developed countries with the result that the respiratory systems of healthy children come under attack, Professor Locksley said, adding: "The marked increase in asthma in developed countries may represent the price we pay for shoes and concrete, which have eliminated the usual pathogen [disease] target and the timing during which this type of immune response would normally become directed to the gut."

In the past 20 years, asthma reports have increased fourfold in Britain. Although this might be due to better diagnosis by doctors, it is also linked with central heating and the house-dust mite.