The World Health Organization says that two sisters in Vietnam died 10 days ago of the H5N1 avian-influenza virus and that they may have contracted it from their brother. If so, this could be the first known instance of the virus being passed from human to human, but the transmission path may never be conclusively identified. The virus is now affecting large parts of Asia, and health officials fear that it could mutate and be transmitted from human to human. However, the virus has been present in Asian chickens since at least last April, which indicates that it has not done a lot of transmitting to humans.