The fifth case of human bird flu in two weeks in Vietnam has raised fears of a new epidemic.
Millions of birds have been culled as a preventative measure
A 35-year-old woman is reported to be in a critical condition, after being taken hospital in Ho Chi Minh City. The four earlier cases have been fatal.
Bird flu swept through Asia last year, killing more than 30 people and leading to culls of poultry in 10 countries.
A World Health Organization (WHO) expert has warned that many more could die if the virus is not contained.
"If strong measures are not in place, the epidemic can spread to an unmeasurable extent," Hans Troedsso was quoted as saying at a conference in Hanoi on Thursday.
The latest victim, from Vietnam's southern Mekong Delta province, had reportedly been hired by a neighbour to bury dead ducks and to pluck sick ducks' feathers for sale.
She developed a fever and breathing difficulties a few days ago, doctors said.
The outbreak in Vietnam was first reported last month, when the authorities said some 4,000 chickens had either died or been culled in the south of the country.
More than 100 million birds have died or been killed around Asia last year because of the bird flu virus.
The virus has killed 25 people in Vietnam and 12 in Thailand over the last 12 months.
Experts have warned of the risk of a world pandemic if bird flu combines with a human flu virus.
The WHO on its website said the recent tsunami that devastated parts of South East Asia may have raised the risk of a pandemic.
It said preventative measures need to be taken across the region to ensure the avian influenza is not imported to those areas made vulnerable by the tsunami.