At least six people have died and one other was sickened in the largest of four such clusters to date in which the virus has infected multiple members of the same family.
"We believe she may have had some contact either with dead or dying chickens in her household or through her activities as a vegetable grower and a seller in a market," said Steven Bjorge a WHO epidemiologist in Jakarta.
The WHO has said it is possible the disease may have spread through limited human-to-human transmission in the latest cluster of cases, but it doesn't appear to have spread outside the family.
Earlier, health officials said they had no evidence that anyone in the cluster had contact with poultry infected with the H5N1 bird flu virus, and tests on birds in the village of Kubu Sembelang, where the family lives, all came back negative.
Global and U.S. health officials say samples taken from family members do not indicate the virus has mutated. Experts are closely monitoring the H5N1 virus because they fear a genetic change could enable it to spread easily from person to person, possibly sparking a pandemic.
In other developments:
So far, most human cases have been traced to direct contact with infected birds.
Bjorge said the family members were in close physical proximity while they were sick, including sleeping near one another.
He said there is no reason for alarm because rare cases of human-to-human transmission have been observed previously.
"Even though so many people were tragically affected in this case, it hasn't really changed the picture of avian influenza in Indonesia at this time," he said.
Bjorge said a team of international health experts and local villagers themselves are closely monitoring the area in northern Sumatra to ensure no one else experiences flu-like symptoms.
"We want to find out if there is any possibility of even one person having mild symptoms that might have been overlooked," Bjorge said. "We are very concerned about this large outbreak and we've taken it very seriously as has the government."
Local authorities have resisted working with outside health experts and many villagers blame black magic, not bird flu, for the deaths of the family members.
Though other cases of limited human-to-human transmission have been documented — including a mother and child in Thailand — this may be the first time bird flu has been passed among multiple individuals, Dr. Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention, said Wednesday.
Bird flu has killed 124 people worldwide, more than a quarter of them in Indonesia.
It remains unclear exactly how the virus spreads among family members — whether it is through respiration, food, contact with infected surfaces or a combination of these vectors, said WHO spokesman Gregory Hartl in Geneva.
Other experts have suggested certain family members could have a genetic weakness to the disease. In all, four family clusters have been recorded so far, and only direct blood relatives — not spouses — have contracted the disease.
The WHO said it will leave its pandemic alert level unchanged at 3, where it has been for months, meaning there is "no or very limited human-to-human transmission."
In the latest family cluster, the first woman to die was buried before samples could be collected, but she was considered to be among those infected with bird flu.