Asthma, which affects more than three million people in the UK, is caused by an allergic reaction.

The number of cases in Britain has trebled in the past 20 years, suggesting that the condition is exacerbated by modern-day living.

Until recently, air pollution was blamed. But in places like Beijing, in China, where the air can be 35 times dirtier than here, asthma levels are much lower.

Experts now believe the secret could be the modern-day obsession with hygiene.

Standards of cleanliness have become so high, they believe, that the body's immune system is not exposed to germs, and therefore does not develop the ability to fight off infection in the way that it should.

Good bacteria

Getting muddy may not be such a bad thing

Doctors at University College Medical School in London have been researching the links between bacteria and the immune system for the past 20 years.

They have developed a vaccine based on this theory.

It works by introducing a group of organisms to the body which act as "good bacteria" and restore the balance in the immune system.

The trials are being carried out at the University of Southampton, which has Europe's largest single allergy and asthma research unit.

Dr John Stanford, a microbiologist who helped develop the vaccine, said: "We are quite confident that we can use this to reduce the amount of other treatments that are required to control asthma and eventually, perhaps, to prevent asthma starting.

"Cure is a very difficult thing to define and it's too early to say if that can be done."

Britain currently tops the European league table of asthma sufferers and in some cities 40-50% of children suffer from some form of allergy.