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Public fears of bird flu seem to have abated in recent weeks, but scientists know the world is always one viral mutation away from a deadly pandemic. That fact has been driven home again by a worrying cluster of human bird flu cases in rural Indonesia that could represent the first time the H5N1 virus has managed to pass from human to human to human. The cluster likely began with a 37-year-old woman who hosted a family pork roast on April 29 in the Indonesian village of Kubu Sembilang in north Sumatra. The woman had become sick on April 27, and as she worsened, several family members slept in the same small room as she did. By the first week of May six more members of the family had fallen ill with avian flu. The first woman died on May 4 and was buried before any tissue samples could be taken, but doctors were able to confirm H5N1 in the remaining family members, all but one of whom have died. An eighth family member, a 32-year-old man, became sick on May 15 and died May 22; he may have caught the virus while caring for his infected 10-year-old son, who died of the disease on May 13.

The WHO has dispatched a team of investigators to the area, including experts from the organization's headquarters in Geneva, but they have been unable to find any evidence of contaminated poultry in the village that may have triggered the human infections. "If we can't find an external source that explains all seven confirmed cases, then we have to go with the theory that this is human-to-human," says Peter Cordingley, the spokesperson for the WHO's Western Pacific regional headquarters. Human-to-human transmission within a family is believed to have occurred at least twice before, in Thailand and Vietnam, although never involving this many people. But if the 10-year-old boy was infected by a family member, and then went on to infect his father, it would represent the first known time the virus had passed from human to human to human. "It's certainly possible," says Gregory Hartl, a WHO spokesperson in Geneva.

That's a worrying threshold to cross, but the good news is that the virus doesn't seem to have spread outside the family. The 32-year-old man ran away from doctors after falling ill, and passed though four neighboring villages before he was apprehended, coming into contact with 33 people. All of them are currently under observation and being given the antiviral drug Tamiflu as a prophylactic, but none have shown signs of infection. Scientists have genetically sequenced two viruses isolated from the cluster and found no evidence of the kinds of significant mutations that would likely be necessary before the virus could pass easily from person to person. "The virus looks pretty much the same as other cases," says Dr. Guan Yi, an avian-flu expert at the University of Hong Kong who has seen the genetic sequences.

If those contacts remain healthy for the next week and a half, then the outbreak at Kubu Sembilang will likely be judged contained. But the cluster itself could remain a mystery. Villagers have been extremely uncooperative with investigators, complicating efforts to get samples from animals and forcing the WHO to set up its command hub 5 miles from the village. "We don't have a lot of access to the village right now," says Hartl. "But they've lost seven people. There's a lot of shock and grief they have to work through first." It's a reminder of the power bird flu still has to surprise—and to kill.

• BRYAN WALSH/HONG KONG
• Eight family members in rural Indonesia fall ill, and medical investigators are racing to figure out why