We must capitalise on this breakthrough

Dr John Harvey, British Thoracic Society

Researchers from the John Radcliffe Hospital in Oxford now believe they know why some babies are affected far more powerfully than others by the virus.

Carriers of a particular genetic variant appear to have twice the risk of a serious infection.

Scientists already knew that the walls of airways infected with the respiratory syncytial virus, or RSV, seemed to have a large amount of a gene called IL-8.

This gene exerts a powerful effect on the way the body responds to infection by causing inflammation in the tissues involved.

The more inflammation there is, the more severe will be the respiratory symptoms triggered by the virus.

DNA tests

The research team took DNA swabs from the mouths of families whose children had been admitted to hospital with severe bronchiolitis between 1992 and 1999.

They wanted to see if they carried a variant of the IL-8 gene which might be causing this more powerful inflammatory response.

They found that this variant was present in a great number of the families.

Blood taken from the umbilical cords of 180 normal babies suggested that the variant gene is probably present in about half the population, with carriers at twice the risk of developing severe bronchiolitis.

Dr John Harvey, from the British Thoracic Society, which represents chest physicians, described the discovery as a "huge step forward"

He said: "The virus infects nearly all children by the end of their second winter but why some go on to develop bronchiolitis has never been fully understood.

"We must capitalise on this breakthrough and conduct further research to understand exactly what role this gene plays."