Scientists have discovered a potential reason to explain why the H5N1 strain of bird flu is so much more deadly to people than standard human flus.
The H5N1 virus was shown to trigger a potentially fatal immune reaction
A team in Vietnam compared people infected with the different flus.
The Nature Medicine research found that the bird flu virus triggers a massive inflammatory response, which often proved fatal.
A UK expert said the study provided vital information about how best to treat people infected with the virus.
There have been 241 cases of people being infected with H5N1 since the outbreak started in 2003. Over half died from the disease.
The team from the Oxford University Clinical Research Unit at the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, looked at 18 people who had had H5N1 and eight who had had normal human flu.
They looked at the level of the viral load - the concentration of the relevant virus in a person's blood, and at how the person's immune system had responded to infection.
It was found that the patients infected with H5N1 had much higher viral loads in the throat than those patients infected with the human flu virus.
And the markers of viral load were highest in the H5N1 patients who had died.
Virus could also frequently be detected in the blood of H5N1 patients, but only in those who died.
The researchers noted that the presence of high levels of H5N1 virus triggered a release of proteins called cytokines which should control a body's response to infection.
The highest levels of cytokines were seen in those with the highest viral loads- who were those who had died.
In these cases there was also an associated loss of lymphocytes (types of white blood cell) in the peripheral blood.
The team suggests it is these factors which lead to lung damage and, on many occasions, death.
Dr Menno de Jong and his colleagues wrote in Nature Medicine: "The focus of clinical management should be on preventing this intense cytokine response, by early diagnosis and effective antiviral treatment."
Professor John Oxford, a virus expert based at Barts and The London NHS Trust in the UK, said: "This clearly puts the emphasis on the level of virus a person has.
"The higher it is, the higher the chance of death."
He said it showed people infected with H5N1 should be treated with antiviral drugs - Tamiflu, Relenza or amantadine - in order to reduce the amount of virus in their systems.
Professor Oxford said the information would also help if the virus mixed with a human flu and mutated into a form which was easily transmitted between people.
He added: "We are lucky this is happening now. If it had been the 1970s or 80s, we would not have had these antiviral drugs to turn to."