The only remaining member of a family killed by bird flu is treated in Indonesia's Northern Sumatra province.


However, there were concerns the virus may have been spread by human-to-human contact, he said.

"We have not had a cluster as large as this -- seven people in an extended family," Cordingley said.

"We can still find no sign of any sick animals that might have infected these people so we've got a puzzle on our hands and it's a worrying one."

All seven members of the family have since died. (U.N. agency launches probe)

Health officials have long been concerned about a mutation in the bird flu virus that would make it spread more easily among humans.

Tests done by WHO scientists show that this has not happened.

"There is no change in the virus whatsoever," Cordingley said. "This virus has not developed the ability to jump more easily from chickens to humans, nor spread among humans more easily."

According to Cordingley, the people were living together in very cramped quarters -- seven people in one family, many living on one room.

"All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness," a WHO statement said Tuesday.

"Although human-to-human transmission cannot be ruled out, the search for a possible alternative source of exposure is continuing," it said.

The H5N1 virus has infected humans in 10 countries across Asia and Africa.

The WHO has confirmed 218 human cases worldwide; of those, 124 died.