A 26-year-old man who worked as a nurse in a hospital treating bird flu patients in northern Vietnam has contracted the deadly virus.
A doctor says it is still unclear whether the man contracted the virus from a patient or through contact with sick poultry.
Further investigations will be carried out.
"He is the nurse who took care of a patient confirmed positive to H5N1," a doctor from Hanoi's Institute of Tropical disease said.
The patient, aged 21, from northern Thai Binh province, has been in critical condition for the past 10 days.
A 69-year-old man from the same province of Thai Binh, 100 kilometres from Hanoi, died of bird flu on February 23.
Four others are currently in hospital in Hanoi, with three said to be in a stable condition.