During the last months of 2003, outbreaks of avian flu caused by influenza A (H5N1) virus were described in eight Asian countries, and has paralleled an epidemic of human influenza caused by the same avian virus A (H5N1). Between December 26, 2003 and October 13, 2005, there had been 117 documented human infections and 60 deaths (mortality, 51%) in four Asian countries: Vietnam, Thailand, Cambodia and Indonesia. Human influenza sustained by H5N1 virus is usually characterized by a severe lower respiratory tract infection, leading to a progressive respiratory failure and several extra-respiratory complications. Infected birds have been the primary source of influenza H5N1 infection in humans in Asia, but transmission from poultry to humans is very limited at present, and requires a direct exposure to live birds, whereas there was no significant risk related to eating well-cooked poultry meat. Human-to-human transmission of H5N1 virus has been suggested in several household clusters, but it seems very uncommon at present. All the viral genes of human H5N1 isolates were of avian influenza origin, which indicates absence of reassortment with human influenza viruses, but H5N1 continue to evolve and persistent monitoring is requested to identify any increase in viral adaptation to man, which could lead to a human pandemic.