The way asthma works has long been known, but why it starts in the first place has been difficult to pin down, leading to several theories on the matter.

But an international team based at St George's Hospital Medical School in London has discovered how dust mites break through the body's protective layers to cause an allergy and thus asthma.

Their findings suggest that an enzyme in dust mite faeces penetrate can destroy the biological "glue" holding protective cells in the airways together.

This leaves the layer riddled with holes, allowing the particles to come into contact with lung tissue.

Crucial events

Dr Clive Robinson, leader of the international team that made the discovery, said the finding offers hope for the treatment of all allergies, but it was the application to asthma that was most exciting.

A spray may be available in five to seven years

He told BBC News Online: "Our discovery uncovers for the first time one of the very earliest events that occurs in the lungs of people who go on to develop asthma."

"That's why we're very excited about the prospect of preventing the disease occurring in people who are at risk."

Although a certain cause for asthma has proved elusive, doctors are clear on the workings of the disease.

The lungs and airways become oversensitive to particles in the air, which leads to "attacks", or acute episodes, when the air passages narrow and breathing becomes difficult.

Growing problem

One in seven children now suffer from the disease in the UK and it is on the rise worldwide.

Factors known to be involved include pollution and allergies such as hayfever.

Dust mites have long been known to contribute to triggering an attack, but it was not known why some people were allergic to them while others were not.

The St George's team discovered how dust mite faeces break through the body's defences to attack lung tissue, allowing them to trigger an allergic response.

The discovery will allow doctors to work on new medicines, although they may not be on the market for some time.

"It will be between five and seven years before we have a medicine in a bottle or a spray," Dr Robinson said.

"But we know we can stop the process we've discovered in the laboratory, so we're very hopeful we can change that into a useful treatment."

Prevent and cure

He said two groups of patients would benefit - those who were at high risk of asthma could take medicine to prevent the condition developing, and those who already had it could take medicine to control and possibly even reverse it.

"We suspect that you might be able to just treat people within a window of opportunity when they are in early life and vulnerable to developing dust mite allergies," he said.

"Once the person is through that period it may be possible to actually end the treatment."

The team is now looking for funding from pharmaceutical companies to work on developing a useable medicine, or they may form their own company.