The old man from the northern province of Thai Binh, where a cluster of cases is causing great concern about the possibility of human-to-human transmission, drank duck blood during the Tet Lunar New Year last month, health officials in his village said.

"It is really strange that he carries the virus for a month and did not get sick," said one health official in the Thuy Luong commune of Thai Thuy district of a virus that usually takes three to 10 days to show symptoms.

However, there was no immediate confirmation from experts that the old man had indeed been carrying the H5N1 virus for nearly a month without showing symptoms, although his 21-year-old grandson caught it after drinking raw duck blood at Tet.

The young man´s 14-year-old sister was also infected after coming into contact with sick chickens and health officials confirmed this week that a nurse who tended the grandson had caught bird flu, which has killed 47 people in Asia.

Top medical officials said there was evidence the 26-year-old male nurse lived in an area where the H5N1 virus had struck poultry and that he had eaten chicken.

But they said the focus of their investigation into the case was the possibility he had caught it directly from the grandson.

What worries the experts most is the possibility of the virus mutating into a form which could pass easily between humans and set off a pandemic in a world without immunity to it and kill millions.

So far, there has been no evidence that mutation has occurred and the only previous probably case of human-to-human transmission occurred in Thailand, where a mother died after cradling her sick daughter in her arms for hours.

The male nurse, being treated in a Hanoi hospital with a high fever, a symptom of the H5N1 strain that has killed more than 70 percent of the people known to have caught it, had provided bed care to the grandson.