An Indonesian man who died of H5N1 bird flu caught it from his 10-year-old son, the first laboratory-confirmed case of human-to-human transmission of the disease, according to a World Health Organization investigation of an unusual family cluster of bird flu cases.
    The investigators also found that the virus mutated slightly when the son had the disease, although not in any way that would allow it to pass more readily among people. Flu viruses like H5N1 mutate constantly, although most of the mutations are insignificant biologically; that appears to be have been the case in the Indonesian cluster.
    "Yes, it is slightly altered, but in a way that viruses commonly mutate," said Dick Thompson, a spokesman for the World Health Organization in Geneva, describing the findings, which were not publicly released. "But that didn't make it more transmissible, or cause more severe disease."
    The greater importance of the slightly modified virus is that it allowed researchers from the organization and the United States Centers for Disease Control to document for the first time that the virus almost certainly passed from the son directly to his father.
    In previous cases where human-to-human transmission was suspected, scientists were not able to say for sure, either because test samples from the patients were not available or because the virus in the patients was the same as that found in poultry in the area.
    Scientists say the H5N1 virus, which has killed hundreds of millions of birds worldwide, does not spread easily to humans or among them. But they have worried that it might, through normal biological processes, acquire the ability to do so, potentially setting off a devastating human pandemic.