The 81-year-old man first tested positive five days ago in Thai Binh province, before the diagnosis was confirmed at Hanoi's Institute of Epidemiology.

``He has never shown any clinical symptoms of the disease,'' Dao Trong Bich, deputy director of the district medical center, said. The man was the third person in his family to test positive for the deadly H5N1 virus.

His 21-year-old grandson has been in critical condition for the last two weeks while a granddaughter, 14, remains in hospital but in good condition.

It was not clear whether the elderly man, Nguyen Huu Kim, caught the disease from his grandchildren or infected poultry, but the possibility of a human-to-human infection will spark concern among health authorities.

Experts fear a worldwide pandemic if bird flu mutates to be easily transmissible between people.

Officials said Monday a male Vietnamese nurse who cared for the grandson has also contracted the virus. The source of his infection has not been determined either.

A doctor from Thuy Luong commune, where the family lives, said the brother and sister in hospital live with another sister and their parents, who appear free of the virus. The grandfather lives in another house in the same village, where poultry stocks have been hard hit by bird flu.

According to the family, Kim had several sick poultry. Some were killed and eaten during the Lunar New Year holiday in early February.

``I'm very surprised to know that my two grandchildren and myself have been infected with bird flu,'' Kim said. ``I'm very strong, not sick at all. I eat and sleep well.''

No one else in the village is known to have been infected.

It is not the first time members of the same family have become infected in Vietnam. Earlier this year, two brothers from Thai Binh were hit by the flu, and one died.

Also Wednesday, an official from Vietnam's National Institute for Hygiene and Epidemiology said another case of an H5N1 carrier showing no symptoms of flu has likely been identified. She is the widow of a man who died of bird flu on February 23, also in Thai Binh.

Her case and that of the 81-year-old man have fueled fears that H5N1 is more widespread than thought.