The World Health Organization says it cannot rule out limited human-to-human transmission of the H5N1 bird flu virus, which killed seven members of one Indonesian family earlier this month.
Six of the seven family members were from one village in north Sumatra.
WHO regional spokesman Peter Cordingley in Manila says there have been groups of people infected with bird flu before, but this is the largest number in one location.
"We have seen clusters in the past and we have seen what was almost certainly (maybe) limited human to human transmission in some cases," he said. "But this is significant because it's the mother of all clusters. It's seven people, six of whom have died and possibly an eighth as well who died before samples could be taken so this is the mother of all clusters."
Cordingley says the WHO is particularly concerned because scientists cannot find a link to sick birds in these latest deaths.
"For the first time now we have people dying from this virus but we can find no source of infection outside their own family," he explained. "No dead chickens, no dying chickens, basically no animal source at all around them. So we're in a zone we've not been in before which is a large cluster and we don't understand it."
So far no human-to-human cases of avian flu have been confirmed.
And Cordingley stresses there is some positive news in this Indonesian cluster.
"We've taken samples and we've looked at them and the virus is not mutating…it shows no sign of the ability to transmit more easily between chickens and humans and no sign of any ability to transmit more effectively from human to human," he said.
Scientists fear if that happens it could cause a pandemic killing millions of people.
Bird flu has infected more than 120 people worldwide - the majority of them in Asia and a quarter of those in Indonesia - since it resurfaced in 2003.