World health experts fear seven members of an Indonesian family struck by bird flu may have infected each other. It is one of the largest outbreaks of the disease among humans. The World Health Organisation says although there is no indication yet that the virus has mutated into a form that can be easily transmitted among people, it is still a worrying development.Peter Cordingly of the WHO said: “We are worried for two reasons. One is that we have never seen a cluster this size before. And the second reason is that we have a team down there, they are examining what is going on and they can’t find an animal source of this infection and that worries us. And in the absence of a known animal source, we have to treat this as possibly human-to-human transmission of the virus.” The outbreak occurred in an isolated village in the north of Indonesia’s Sumatra island. The country is one of the worst-affected by bird flu. Scientists fear that if the H5N1 virus were to mutate, it could trigger a global pandemic. It is thought cases of person-to-person transmission have already happened as a result of close and prolonged contact. But it appears to be on a much larger scale this time.