Researchers at the University of Iowa think that the virtual elimination of such creatures from the human gut over the years has left the body vulnerable to the massive immune reactions which typify the conditions.

Although only six sufferers took part in this trial, the results were so impressive that larger experiments could now follow.

All six were given a drink containing microscopic worms which can survive, although not reproduce in the human gut.

Between two or three weeks later, their symptoms completely disappeared, and stayed away for about a month.

A sterile existence

Dr Joel Weinstock, who carried out the tests, said: "We're living in boxes, breathing sterile air and drinking sterile water.

"As we've de-wormed, people have developed immune systems which are not damped.

"The worms living in the gastrointestinal tract have been with us for three million years, and our immune systems have got used to their presence."

Crohn's disease, ulcerative colitis and other forms of inflammatory bowel disease appear to be cause by an overactive immune system, which causes inflammation in the digestive system.

Symptoms include abdominal pain, bowel obstruction and bleeding.

The condition is incurable, and normal treatments include steroids, which can reduce the inflammation, although these have been known to produce side effects.

Dr Weinstock pointed out that the rise in such diseases over recent years has coincided with a reduction in the incidence of parasitic worms in humans.

Four tenths had worms

As little as 70 years ago, he said, 40% of US children enjoyed the company of worms which could grow up to 20 centimetres long.

Dr Mark Cottrill, a Lancashire GP with a special interest in IBD, said the use of worms was certainly novel.

He said: "I'm always open-minded about any innovation, even though treatments have become much better."

He said that he occasionally found harmless worms living in peace in his patients when he examined them with a colonoscope.

He said: "You do see these white things which don't like the light and wriggle away."

Other scientists, such as Dr Balfour Sartor, from the University of North Carolina, are experimenting with the use of bacteria to damp down the immune system in IBD patients.