A family of eight infected with bird flu in Indonesia likely passed the disease among themselves, but world health officials said Wednesday there is no reason to raise its pandemic alert level.

It is the fourth — and largest — family cluster of bird flu cases likely transmitted from person to person since the start of the outbreak in Hong Kong in 2003, World Health Organization spokesman Gregory Hartl said.

But this case may mark the first time bird flu has passed from person to person to person, a top U.S. health official said.

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The previous clusters all involved someone who was infected by a sick bird and then spread the virus to others. This new cluster appears to involve a cascade of transmission, said Dr. Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention, in a telephone briefing from Geneva.

The family members' close physical proximity is probably responsible for the spread of the disease, Hartl said.

"It fits the kind of pattern perfectly which we've seen so far," Hartl told The Associated Press. Global and U.S. health officials say tests on virus samples taken from the family do not indicate any significant changes.

Investigators say the family, living in the remote northern Sumatra village of Kubu Sembelang, were infected with a strain of H5N1 bird flu that was genetically the same as the virus found circulating in the area earlier. Tests are still being carried out on poultry there.

Infections have not been detected in health-care workers or others in the village, but those who may have come in contact with the family are being given Tamiflu as a precaution, Gerberding said.

WHO has suspected that in rare cases bird flu may have passed from one person to another, although people usually catch it from chickens and other poultry.

Experts have long believed the virus is spread when people breathe it in — possibly in dust from bird droppings or in droplets sneezed or coughed by humans into the air.

But it remains unclear exactly how the virus spreads in family groups — whether through respiratory systems, food, infected surfaces or a combination of these, Hartl said.

"Which transmission mode is most important, we really don't know yet," he said. "When you get all of these things together, it becomes perhaps more likely."

Other experts have suggested family members have a genetic weakness to the disease. In all four family clusters recorded so far, only direct blood relatives — not spouses — have caught bird flu.

WHO 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."

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.

"... It apparently can only spread between human to human when there is extensive and close contact between someone who is already showing clinical signs of the disease and the uninfected person," Hartl said.

In the three previous family clusters — in Indonesia, Thailand and Vietnam — the number of infected relatives was much smaller, Hartl said. CDC officials noted that the evidence for human transmission was stronger in the Indonesia and Thailand clusters, but human transmission could not be ruled out in the Vietnam cluster.

Health workers have found no sign the latest Indonesian case has moved outside the family, and there is also "no evidence that efficient human-to-human transmission has occurred," WHO said in a statement.

Still the size of the cluster and the failure to determine the source of the infections was worrying, Peter Cordingley, spokesman for WHO's Western Pacific region.

Bird flu has killed 124 people worldwide since the virus started ravaging poultry stocks across Asia in 2003.