Avian influenza (bird flu) is a contagious disease of animals caused by influenza A viruses. Since their reemergence in 2003, highly pathogenic avian influenza A (H5N1) viruses have reached endemic levels among poultry in several southeast Asian countries and have caused human infections with high mortality. Since october 2005, H5N1 was confirmed in poultry in Turkey. Outbreaks are continuing with varying degrees of severity in several affected areas. Therefore, influenza H5N1 is an important threat for human being. The current knowledge of the virology, clinical spektrum, diagnosis and treatment of human influenza H5N1 virus infections is reviewed herein.