In the wake of a cluster of avian flu cases that killed seven members of a rural Indonesian family, it appears likely that there have been many more human-to-human infections than the authorities have previously acknowledged. The numbers are still relatively small, and they do not mean that the virus has mutated to pass easily between people – a change that could touch off a worldwide epidemic. All the clusters of cases have been among relatives or in nurses who were in long, close contact with patients. But the clusters – in Indonesia, Thailand, Turkey, Azerbaijan, Iraq and Vietnam – paint a grimmer picture of the virus’s potential to pass from human to human than is normally described by public health officials, who usually say such cases are ‘rare.’ Until recently, World Health Organization representatives have said there were only two or three such cases. On May 24 Dr. Julie L. Gerberding, director of the federal Centers for Disease Control and Prevention in Atlanta, estimated that there had been ‘at least three.’ Then, last Tuesday, Maria Cheng, a W.H.O. spokeswoman, said there were ‘probably about half a dozen.’ She added, ‘I don’t think anybody’s got a solid […]