A big upsurge in asthma cases may be partly due to the good standards of hygiene and clean food common in developed countries, researchers have found.

Some scientists believe that exposure to some infections in early childhood may help to prevent the development of allergic reactions, such as asthma and rhinitis, an inflammation of the lining of the nose.

This is because the immune system becomes accustomed to dealing with foreign invaders. Without that exposure, the immune system remains weak and vulnerable.

The theory is hotly disputed by some, but research by Italian scientists published in the British Medical Journal appears to bear it out.

Dr Paolo Matricardi and colleagues in Rome investigated the impact of exposure to two types of bugs:

• Those that are carried in contaminated food and transmitted orally such as H pylori and the hepatitis A virus.
• Airborne microbes such as those that cause measles, mumps and chickenpox

Their research centred on Italian airforce cadets aged 17-24 years. Half were prone to allergic reactions and half were not.

They found that people who had been more exposed to microbes transmitted orally were less likely to suffer from respiratory allergies.

Exposure to airborne microbes appeared to make little difference.

The researchers say that exposure to bugs such as H pylori and Hepatitis A does not in itself prevent allergies.

Instead, by infecting the gut, they may stimulate and strengthen the immune response, and therefore help to protect the body from other invaders.

More research required

However, they warn that further studies are required to verify their findings.

Dr Matricardi said: "We must improve hygiene to reduce the impact of infectious diseases, but at the same time, we must learn how to safely 'train' our immune system, especially during infancy, in order to prevent allergy."

Amanda Broatch, press and public relations manager for the National Asthma Campaign, said: "This piece of research confirms our suspicions that lifestyle factors do contribute to the development of asthma and related allergies."

Professor Dennis Shale, an expert in respiratory and communicable diseases at the University of Wales College of Medicine in Cardiff, said the research echoed previous studies that appeared to show exposure to bugs at an early age was essential to build up the immune system.

He said one theory was that without such exposure the immune system formatted itself in way that made it vulnerable to allergic disorders.

However, Professor Shale warned against parents dropping their guard.

He said: "It would not be sensible to be less scrupulous, I would not encourage parents to let their children go swimming off dirty beaches, or not to wash their hands before preparing food.

"You only have to look at the third world to see the impact that infections can have on children."