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TONY EASTLEY: A Chinese man who died from bird flu passed the virus onto his father in what doctors say is a rare case of human to human infection.

Chinese researchers tested the son and father and found that the strain of virus in both men was identical.

But another 90 people who were exposed to the son, showed no trace of the virus - suggesting that the two men shared a genetic susceptibility to the disease.

The case has been reported in the latest edition of the Lancet medical journal, as Jennifer Macey reports.

JENNIFER MACEY: A 24-year-old man from Nanjing in eastern China died from bird flu last December six days after visiting a poultry market.

His father who nursed him in hospital caught the H5N1 virus but survived.

Dr Jeremy Farrar is an infectious disease expert at the Hospital for Tropical Disease at Ho Chi Minh City in Vietnam.

JEREMY FARRAR: This is a very convincing case, where the son acquired the virus by visiting a poultry market in his home town. His father had no contact with him just prior to that and looked after the son whilst the son was dying in hospital and was therefore exposed to all secretions and whatever that the son was producing as he died.

And therefore that is not normal contact. That's not like me sitting next to you on a bus. This is extremely intimate contact both with patient and with all of their secretions etc.

JENNIFER MACEY: Human to human transmission has been shown before - but patients infected with the disease haven't been as rigorously tested.

Researchers at Beijing's Chinese Centre for Disease Control and Prevention tested samples from both men and found the strain of the virus to be identical.

They also examined another 90 people who had come into contact with the son but they all tested negative for the H5N1 virus.

The study has been published in the latest Lancet medical journal with a commentary from Dr Farrer. He says it suggests the two men were who closely related, shared a genetic susceptibility to infection.

JEREMY FARRAR: In fact of the 370 cases of H5 so far around the world, about 25 per cent of them - getting towards 100 people - have been part of clusters and those clusters invariably have been within blood relatives. So in other words, not husband to wife but father to son, mother to daughter, mother to son, brother to brother. They have been people who are genetically related.

So it does raise the possibility that these particular viruses which are infecting humans are doing so in individuals that are related to each other and perhaps share some common genetic susceptibility to infection with that particular virus.

JENNIFER MACEY: And he says it's not an indication that the world is closer to a pandemic.

JEREMY FARRAR: I find this paper very reassuring, because of 90 odd other very close contacts of the son including his girlfriend who actually did look after him before he went to hospital, none of them got infected. None of them got sick. In fact, none of them showed any evidence that they had been infected with the violence. So it suggests that human to human transmission of this virus remains incredibly difficult.

TONY EASTLEY: Professor Jeremy Farrar from the Hospital for Tropical Disease at Ho Chi Minh City in Vietnam* ending that report by Jennifer Macey.

*Editor's note: This transcript has been amended to identify the correct interviewee.