By Fergus Walsh
BBC Medical Correspondent in Ho Chi
The isolation ward of the Hospital for Tropical Diseases in Ho Chi Minh City stands empty.
Vietnam has slaughtered thousands of birds
They have not had a patient with avian flu here for three weeks.
Doctors hope the warmer weather means this year's outbreak in humans is over.
But large parts of Vietnam are still reporting cases in poultry so no-one is letting their guard down.
The hospital is a mix of old and new. The isolation ward is a relic from the original hospital built by the French in the 19th century.
The unit will be torn down later this year and replaced with modern facilities.
But the research laboratories are in a new block and more spacious than most you would find in the UK.
The battle against bird flu involves a Vietnamese team led by Dr Tran Tinh Hien.
Working alongside them is Dr Jeremy Farrar, director of the University of Oxford Clinical Research Unit, which is part-funded by the Wellcome Trust.
Dr Farrar has been in Vietnam for nine years and well remembers the first patient with bird flu from the outbreak last year.
He said: "I was called at home by Dr Hien the night before Tet began in January 2004.
"Tet is the New Year festival and it's a very big event in Vietnam, and everyone wants to be with their family."
But Dr Hien was worried about a four-year-old girl he had been asked to visit who had severe respiratory problems.
Dr Farrar has seen patients deteriorate rapidly
A hunch told Dr Hien that this was not a case of normal flu.
The girl was admitted to the Hospital for Tropical Diseases and she was treated with anti-virals, which are effective against avian flu if administered early. She survived.
It later emerged that the family kept a pet duck.
"The duck was the girl's pride and joy and she used to cuddle it.
"When it got sick the little girl buried it, but she couldn't bear to be parted with it so she had dug it up again."
In 2004 the hospital treated eight patients with avian flu. Three survived.
Since the beginning of this year it has dealt with nine confirmed cases. All of the patients died.
Nowhere else has seen so many patients with avian flu.
Dr Hien said the disease is incredibly aggressive: "The problem is that most patients are referred when it's too late.
"The hospital has all the equipment and drugs it needs to treat avian flu, but these are of little use if the disease has taken hold."
The course of the disease can be incredibly rapid.
Dr Farrar said: "We saw a patient down at a hospital in the Mekong Delta, who was sitting up in bed having breakfast and was able to tell her story.
"We transferred her to our unit but by the evening she was dead."
And it appears the disease may be more widespread than previously thought.
This follows the deaths last year of two Vietnamese children reported in the New England Journal of Medicine.
A four-year-old boy was hospitalised with severe diarrhoea, but no apparent respiratory problems.
The isolation ward is empty for now
He died of encephalitis (inflammation of the brain), which is common in south-east Asia.
No-one suspected avian influenza but by luck doctors took samples and found the virus throughout much of his body.
It suggests that the virus has the power to attack not just the lungs, but the brain and digestive system.
The boy's nine-year-old sister died in similar circumstances two weeks earlier and it is assumed she had avian flu but no samples had been taken.
Doctors in Vietnam have been advised to be on the alert for severe illnesses or unexplained deaths of any kind, which may now be attributable to avian flu.
Potential for disaster
What the world wants to know is whether avian flu might one day present a global threat.
To do that it would have to be easily transmissible from human to human.
So far, thankfully, that hasn't happened.
There is one probable case - a mother in Thailand is believed to have caught the virus from her sick child - but it involved an unusually high level of contact.
The woman held her dying daughter in her arms for many hours until the girl vomited and died.
But history suggests it is only a matter of time before the next flu pandemic.
The outbreak of 1918 (Spanish flu) killed up to 50m people.
There were further pandemics in 1957 (Asian flu) which killed one million people and in 1968 (Hong Kong flu) which killed 800,000.
It is possible that the H5N1 avian flu virus will not be responsible for the next pandemic, but then it is possible that it will. No-one knows.
Dr Farrar said: "If this virus retained its aggressive, nasty side and developed the ability to go from me to you and through you on your plane back to London, then you are looking at huge numbers of people dying as we probably have no pre-existing immunity to this virus.
"It would be a global disaster."