Jakarta, Indonesia ? The U.N. health agency is looking closely at possible limited human-to-human transmission of bird flu between members of an Indonesian family but said there was no evidence indicating the virus had mutated or that it had spread beyond the relatives.

“We’re not surprised that there is possible human-to-human transmission,” said Steven Bjorge, a World Health Organization epidemiologist in Jakarta, Indonesia. “The thing we’re looking for is whether it’s sustained beyond the immediate cluster.”

Six of the seven people in the family from northern Sumatra who have caught the deadly disease have died, the most recent on Monday. It is one of the largest human clusters ever reported.

Bjorge, who is the team leader at the village in Kubu Sembelang, said none of the poultry in the area tested positive for the H5N1 bird flu virus, which has led a team of international experts to explore whether the virus spread among the relatives.

He warned, however, that such isolated cases of very limited human-to-human transmission have been documented – including a case in Thailand involving a mother and child – and that it does not mean a pandemic flu strain has emerged.