Scientists have discovered the active part of the virus which is thought to be behind a third of all asthma cases.

The respiratory syncytial virus (RSV) inflames the inside of the lungs, weakens the immune system and produces a mucous which obstructs the airways.

Protein

Scientists at the Imperial College School of Medicine in London and American experts at the National Institute of Health believe they have identified the protein which causes the inflammation.

They say only a small amount of the G protein is necessary to damage the immune systems of mice.

The National Institute in Washington hopes to develop a vaccine which could be tested on humans within three years.

Vaccine

Peter Openshaw of Imperial College told the 'New Scientist': "We really do feel we're on the verge of developing vaccines against conditions like asthma that involve manipulation of the immune response."