The first person infected in a cluster of bird flu cases in a family in Indonesia may have come into contact with sick or dead chickens before possibly passing the virus on to relatives, a World Health Organization official said Thursday.

The woman grew vegetables and sold them in a market, which may have brought her into contact with infected poultry, said Steven Bjorge, a WHO epidemiologist in Jakarta. Investigators also haven't ruled out contamination from chicken feces that the woman used as garden fertilizer, he said.

Officials have not linked her family members to any possible exposure to the virus from birds, which has led them to believe that limited human-to-human transmission may have occurred.

Six of the seven family members who caught bird flu have died, the most recent on Monday. An eighth family member who died was buried before tests could be done, but she was considered to be among those infected with bird flu.

The deaths in the family cluster were the largest ever reported. The WHO has stressed the virus has not mutated into a version easily passed between people or shown any sign of spreading outside the family — all blood relatives who had very close contact with each other.

"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," Bjorge said of the first woman infected in the cluster.

He said a team of international health experts and villagers is closely monitoring the area where the family lived in northern Sumatra to ensure no one else experiences flu-like symptoms.

"We are very concerned about this large outbreak and we've taken it very seriously as has the government," Bjorge said. "We want to find out if there is any possibility of even one person having mild symptoms that might have been overlooked."

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.

On Monday, a half-dozen protesters beheaded a chicken and drank its blood to show local authorities that poultry was not the source of the problem.

Tests for the H5N1 virus in birds in the village of Kubu Sembelang have all come back negative, baffling experts.

So far, most human cases have been traced to contact with infected poultry. But there is evidence of isolated cases of limited transmission between people in very close contact with each other.

Scientists are unsure how this has occurred, but they have theorized that the virus may pass from one person to another through droplets sneezed or coughed by humans into the air or food, onto surfaces or in some combination.

It has been suggested that some people may have a genetic susceptibility to the disease. In all four family clusters recorded so far, only direct blood relatives — not spouses — have caught bird flu.

Bjorge said the sick family members in Indonesia were in close physical proximity, which included sleeping close to each other.

"Even though so many people were tragically affected in this cases, it hasn't really changed the picture of avian influenza in Indonesia at this time," Bjorge said.

For more on bird flu, visit Foxnews.com's Bird Flu Center.

A top U.S. health official said Wednesday, however, that the Indonesian case may be the first time bird flu has been passed in a chain of transmission, with a person infected by a bird passing the virus to another person, who then went on to infect a third person or people.

Previous clusters all involved only one jump from person to person. Scientists are still investigating to see how many possible jumps the virus could have made in Indonesia.

Bird flu has killed 124 people worldwide, more than a quarter of them in Indonesia. Scientists fear the H5N1 virus will mutate into a highly contagious form, possibly sparking a global pandemic. So far, most human cases have been linked to infected birds.

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."