The World Health Organisation (WHO) says that despite the cluster of seven members from the same Indonesian family dying of bird flu this month, it does not signify that the virus has mutated and is being transmitted from human to human.

Experts do not believe the cluster is the start of a pandemic as they say there would be hundreds or even thousands of people sick by now.

The WHO says there is no evidence of anyone else being infected though limited human-to-human transmission of bird flu might have occurred within the family and cannot be conclusively discarded or proven.

The WHO has already tested samples from the 6 confirmed cases in the family and the virus looks like the same H5N1 virus that is circulating in birds and for the virus to become one that spreads to humans easily, it has to mutate.

It seems human-to-human transmission is very difficult to prove because epidemiologists have to eliminate all other possible causes, such as the presence of a non-human source or any other infecting agent in the environment.

Firdosi Mehta a senior WHO official in Indonesia says health experts are trying to track down anyone who might have had contact with the family and is putting them on anti-viral drugs as a precaution.

The village in the remote area of Kubu Sembilang in north Sumatra is being closely monitored for any more cases that may occur outside the immediate family cluster.

Although it is the largest bird flu family cluster known to date it is not the first but it has raised concern amongst medical experts who believe clusters raise the possibility that the virus might have mutated to transmit more efficiently among humans that could spark a pandemic that would kill millions.

Mehta says the cluster was no different from other family clusters that have been documented in the past in Hong Kong, Thailand and Vietnam.

Limited human-to-human transmissions of the virus sometimes occurs after very close and prolonged contact when a person takes care of a sick relative who is coughing and probably infectious.

The family are known to have slaughtered and cooked a pig and chickens for a feast on April 29, animals which are known to be highly susceptible to the virus and attempts to test local chickens and pigs for the virus have met with fierce resistance from villagers who blame the government for not providing enough help

This most recent cluster has engendered renewed interest in the theory held by many scientists that genetics might predispose certain people to be infected by H5N1, which remains essentially a disease of birds.

Apparently some people who have survived H5N1 have been found to have more of a type of receptor cells along their respiratory tracts that avian flu viruses like to bind to which in theory would explain why some humans might be more susceptible to H5N1.

Such a genetic trait would also explain why cluster cases have invariably involved blood relations, and never husbands and wives.

The WHO says that familial susceptibility amongst certain races, certain cultures and certain groups of people appears to play a role in the pathogenesis and behaviour of the virus when it jumps from one species, like poultry, to humans.