Domestic ducks may pose a major threat of spreading avian flu to animals and humans, scientists believe.
The team studied mallard ducks
Not only do they harbour the virus with few signs, making it hard to spot, the virus mutates in them, meaning they could cause a large human outbreak.
The International team, including experts from China, Indonesia, Thailand, the US and Vietnam, tested ducks in the lab.
Their findings appear in Proceedings of the National Academy of Sciences.
Millions of chicken and other fowl have died or been destroyed in bird flu outbreaks in several East Asian nations since 2003.
The deadliest strain - H5N1 - has killed 38 people in Vietnam, 12 in Thailand and four in Cambodia since late 2003.
Humans catch the disease through close contact with live infected birds.
Birds excrete the virus in their faeces, which dries and becomes pulverised, and is then inhaled.
To date, the main focus has been on infected chickens. However, more recently cases have been reported in domestic ducks.
Now Dr Robert Webster, from St Jude Children's Research Hospital in Memphis, the US, and colleagues believe these animals have become the "Trojan horse" of the virus - a silent reservoir of infection.
They infected two four-week-old mallard ducks with various strains of the virus isolated in 1997-2004 from humans or poultry throughout South Asia, including H5N1.
Four hours later they placed these in the same cage as two other non-infected ducks.
The researchers observed what happened to the birds over the next 21 days.
Both of the infected ducks shed the virus, mainly through the upper respiratory tract rather than in their droppings.
Both of the other two ducks caught the virus, and one completely cleared it by day seven.
The team found that viral characteristics had changed since 2002.
Ducks infected with H5N1 from 2003 or 2004 shed the virus for 11-17 days, a longer transmission time than pre-2002 strains.
Although the virus no longer caused disease in healthy ducks exposed to it, it was still able to cause disease in chickens.
Dr Webster and colleagues told PNAS: "Here we show that these H5N1 viruses are reverting to nonpathogenicity in ducks.
"Since this newer strain is still potentially harmful to humans, ducks may play a role in spreading further outbreaks."
The real danger is that a mutation will arise that will mean the virus can spread directly between humans.
A spokeswoman from the Health Protection Agency said: "The research offers some interesting new information relating to disease in ducks.
"However, the relationship of this work or implications for human health or human transmission remain speculative, but clearly work in this area is of great interest and reported results are welcomed.
"A primary concern remains the possibility of the virus evolving so that it can transfer readily from human to human, however this is not an issue that this research addresses."
A spokeswoman from the World Health Organization said: "The spread of avian influenza viruses in animals needs to be monitored, as these are viruses that have proven their ability to jump the species barrier."
Dr Juan Lubroth, an expert in avian flu from the Food and Agriculture Organization, said the lab work supported what they were seeing in their field work in Asia.
He said it was important to look at ways of controlling the infection, such as vaccinating healthy animals, culling infected ones and encouraging better farming practices.
For example, ensuring farmers are financially compensated for any animals lost to culling could encourage them to alert authorities to cases of infection, he said.