The spread of the deadly bird flu virus may have been underestimated because of a misunderstanding of how it affects the body, British scientists have said.
The virus is usually passed to humans from infected birds
Oxford University experts studying deaths in Vietnam suggest the disease can attack all parts of the body, not just the lungs as had been thought.
They told the New England Journal of Medicine they also believe humans could pass the virus on to each other.
So far, there have been 42 bird flu deaths, all in Asian countries.
But the Oxford University scientists say their findings suggests the number of cases of human infection with the virus may have been under-estimated.
The World Health Organization said it would change its definition of what constituted a bird flu infection.
So far, the WHO says there have been 55 confirmed cases of bird flu in humans, and 42 deaths.
However, experts believe millions could be at risk if the virus acquires the ability to jump from person to person by combining with a form of human flu to make a new, mutated, version.
The researchers examined the deaths of two young children - a brother and sister - who lived in a single room with their parents in southern Vietnam.
They were admitted to hospital suffering from gastro-enteritis and acute encephalitis, which are common ailments in the country.
Neither displayed respiratory problems, which have been considered typical in cases of avian flu.
But analysis revealed the four-year-old boy had traces of the virus in his faeces, blood, nose and in the fluid around the brain.
Cambodia - 1 case, 1 death
Thailand - 17 cases, 12 deaths
Vietnam - 37 cases, 29 deaths
This indicates the virus - known as H5N1 - can attack all parts of the body, not just the lungs.
It is suspected his nine-year-old sister, who died two weeks earlier in February last year, was also suffering from the virus.
The lead researcher is Dr Menno de Jong, a virologist at the Oxford University Clinical Research Unit who is based at the Hospital for Tropical Diseases in Ho Chi Minh.
He said: "This illustrates that when someone is suffering from any severe illness we should consider if avian flu might be the cause.
"It may be possible to treat but you have to act in the early stages, so awareness of the whole spectrum of symptoms in an emerging disease like avian flu is vital.
"It appears this virus is progressively adapting to an increasing range of mammals in which it can cause infection, and the range of disease in humans is wide and clearly includes encephalitis."
Dr Jeremy Farrar, director of the Wellcome Trust's Vietnam unit, said: "This latest work underlines the possibility that avian influenza can present itself in different ways.
"The main focus has been on patients with respiratory illnesses but clearly that's not the only thing we should be looking for.
"Therefore the number of cases of H5N1 may have been underestimated."
Dr Farrar said the presence of the virus in the faeces suggested that it could be spread from person to person - especially where people are living in crowded conditions.
It is not believed that either of the children passed the virus on, but it is also not clear how they contracted it.
However, the girl often swam in a nearby canal which may have been contaminated by ducks carrying the virus.
Dick Thompson, of the World Health Organization, told the BBC the findings were significant.
He said: "It means the range of illnesses we have been looking for when considering a diagnosis of avian flu will now be expanded.
"We will have to change the way we conduct our investigations, the management of hospital patients and even the way we deal with their bodily secretions."