Highly pathogenic avian influenza H5N1 previously known to infect only birds was also found to infect human, causing disease and death. Continuous outbreaks of the highly pathogenic H5N1 avian influenza A resulted in an urgent effort to improve treatments, vaccines, and diagnosis to lower the threat of an influenza pandemic.Control measures and continuous surveillance aimed at reducing exposure of human to potential H5N1-infected poultry. Pandemic human influenza viruses have been emerging for many centuries, thrice only in last century. These pandemics were caused by the most common subtypes of circulating viruses in the community at that time. The influenza virus has segmented genome, which undergoes continuous mutations and genetic reassortments.Phylogenetic analyses of the H5N1 viruses isolated from humans showed that these viruses were identical to those circulating in poultry. Any genetic change in H5N1 enabling human to human transmission may lead to a pandemic of human influenza.