Scientists have found the first clue as to why some people are hospitalized and die when infected with the H1N1 virus, while others only have a mild form of the pandemic flu.

A new study by Canadian and Spanish researchers shows the difference was related to the amount of interleukin 17 in a patient's blood.

There were high levels in patients with severe cases of H1N1, while those with mild symptoms had low levels.

Interleukin 17 helps regulate white blood cells, which fight infection and disease.

The finding could lead to development of a drug to prevent the severe form of H1N1.

"In rare cases, the virus causes lung infections requiring patients to be treated in hospital," said Dr. David Kelvin, of the Toronto General Hospital Research Institute.

"By targeting or blocking interleukin 17 in the future, we could potentially reduce the amount of inflammation in the lungs and speed up recovery," he said, adding such treatment would be years away.

A test to determine who has high levels of the molecule is possible in the near future, Kelvin said. "A diagnostic test could let us know early who is at risk for the severe form of this illness quickly," he said.

High levels of this molecule would indicate a failure of the immune system to eliminate the virus, similar to what happened during the 1918 Spanish flu when huge numbers of deaths occurred due to an already deadly influenza A virus strain of subtype H1N1.

The study is published in the journal Critical Care.