Laboratory testing has completed full genetic sequencing of two viruses isolated from cases in this cluster. That has found 'no evidence of genetic reassortment with human or pig influenza viruses and no evidence of significant mutations,' WHO said.

"All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness," said a WHO statement. "Although human-to-human transmission cannot be ruled out, the search for a possible alternative source of exposure is continuing."

The agency did not immediately indicate whether it was raising its alert level, which remained early Wednesday at 3 where it has been for months. That means there is "no or very limited human-to-human transmission."

The agency has suspected that in rare cases bird flu may have passed from humans to humans, but it usually has been caught by people from chickens and other poultry.

No Significant Mutations

WHO said that testing indicated there had been no significant mutations in the virus. Experts have feared that a mutation of the virus into a strain that could easily pass among humans could set off a deadly flu pandemic.

According to the WHO, 218 people have been confirmed to have been infected with bird flu since 2003, and 124 of them have died.

The agency said the Indonesian Health Ministry had confirmed a man who died May 22 had been infected with the deadly H5N1 strain of bird flu.

He was the seventh member of an extended family confirmed to have become infected. An eighth person in the family, who died of similar symptoms May 4, was buried before tissue samples could be taken, so the cause of death could not be determined, but she is assumed to be part of the cluster, WHO said.

The family lives in the Kubu Sembelang village, Karo District, of North Sumatra.

"The newly confirmed case is a brother of the initial case," WHO said. "Specimens were taken on 21 May and flown the same day to Jakarta. Tests run overnight confirmed his infection. His 10-year-old son died of H5N1 infection on 13 May. The father was closely involved in caring for his son, and this contact is considered a possible source of infection."

Close and Prolonged Exposure

It said the investigation is continuing, but that preliminary findings indicate that three of the confirmed cases spent the night of April 29 in a small room with the first woman infected and that she was coughing frequently.

That group included the woman's two sons and a second brother, who is the sole surviving case among infected members of this family, WHO said. Other infected family members lived in adjacent homes.

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

So far health workers have found no sign that the case has moved outside the family and there is also "no evidence that efficient human-to-human transmission has occurred."

Laboratory testing has completed full genetic sequencing of two viruses isolated from cases in this cluster. That has found "no evidence of genetic reassortment with human or pig influenza viruses and no evidence of significant mutations," WHO said.

Such a change could have been dangerous, because it might combine the bird flu virus with a strain that also would make it easily pass among humans.

WHO said the viruses also showed no mutations associated with resistance to Tamiflu, the drug on which experts are pinning their hopes to slow or stop the development of a flu pandemic.

Tamiflu can be used to treat infected people and be given to people exposed to the disease.

"The human viruses from this cluster are genetically similar to viruses isolated from poultry in North Sumatra during a previous outbreak," WHO said.