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."