The number of cases of bird flu in humans may have been hugely under-reported, a Swedish study says.
The team quizzed nearly 46,000 people in Vietnam
Doctors quizzed nearly 46,000 people from affected areas of Vietnam, where there have been 87 cases of bird flu.
The Archives of Internal Medicine study found more than 8,000 had had flu-like symptoms and up to 750 cases could have been down to sick birds.
Under-reporting was possible, experts said, but unlikely to be as much as the Karolinska Institute study suggested.
Lead researcher Anna Thorson said the study - the largest one carried out into bird flu to date - clearly suggested the incidence of the virus in humans was much higher than had been recognised.
But she added: "The results suggest that the symptoms most often are relatively mild and that close contact is needed for transmission to humans."
The news comes as the H5N1 virus has claimed its first human victims in Turkey.
It has been confirmed that two children have died in Turkey and reports have suggested more people have been infected by the deadly flu strain which has killed over 60 people in Asia.
The researchers quizzed the people - randomly selected - in the Bavi district of north west Vietnam, which has been hit by bird flu.
Some 8,149 people - nearly one in five - said they had had flu-like illnesses in the pre-ceding months and over 38,000 said they kept poultry.
The team then asked about contact with sick or dead birds and concluded between 650 and 750 cases of flu could be attributed to bird contact.
The researchers said while they could not be certain bird flu had caused the symptoms, it was the only virus poultry was affected by which could be passed to humans at the time.
But Professor Neil Ferguson, an infectious disease expert at Imperial College London, said there were some problems with the research and the indications were that the true level of under-reporting was likely to be lower.
"The conclusions are interesting but not conclusive, as they didn't take blood samples from the people questioned - so they don't really know whether they were infected with bird flu or not."
He added people who did develop flu-like symptoms were more likely to remember contact with dead birds because of the fear over bird flu.