The H5N1 virus has killed 34 Vietnamese, 12 Thais and a Cambodian since it swept across large parts of Asia at the end of 2003. Two elderly Vietnamese, related to family who died of bird flu have tested positive for the deadly virus despite showing no symptoms.
It has recurred several times despite the slaughter of millions of poultry and has spread across half of Vietnam since the latest outbreak began in the Mekong Delta of Southern Vietnam.
Both people live in the northern province of Thai Binh, where a cluster of cases is causing great concern about the possibility of human-to-human transmission of the H5N1 virus, which experts fear could mutate into a form which could cause a pandemic.
A 61-year-old woman's - one of the symptom-free carriers of the virus - only link to the disease was her husband, who died of bird flu on Feb. 24. An official at the health clinic of her village in Quyet Tien commune says she is healthy.
"She took care of her husband when he was sick. Other than that, she said she ate only pork and all four chickens raised in her house tested negative for bird flu."
The other an 81-year-old grandfather of a brother and sister who contracted the disease, drank raw duck blood during the Tet Lunar New Year festivities last month, health officials in the Thuy Luong commune of Thai Thuy say, and find it strange that he has carried the virus for a month and did not get sick. The virus usually takes 3 to10 days to show symptoms, but there has been no confirmation from experts that the man had indeed been carrying the H5N1 virus for a month. The young man's 14-year-old sister was also infected after coming into contact with sick chickens, health officials confirmed this week that a nurse who tended the grandson had caught bird flu.
Top medical officials say they have no evidence that a 26-year-old male nurse living in an area where the H5N1 virus has been found in poultry, had eaten chicken. The main focus of their investigation was the possibility he had caught it directly from his patient.
The main concern for experts is the possibility of the virus mutating into a form which could easily transfer between humans and set off a pandemic in a world with little immunity to it.
At present there has been no evidence that this has happened, the only likely case occurred in Thailand, where a mother died after nursing her sick daughter in her arms. The virus appeared again at the same time in Thailand, where the government said it had spread to 19 of the country's 76 provinces but had not infected any more people. The virus has killed 47 people in Asia.