Three types of influenza viruses, viz. types A, B and C, have been recognised in family Orthomyxoviridae. The influenza A viruses are most important. Influenza A viruses are highly contagious that normally cause respiratory disease in humans, animals and poultry. The avian and swine influenza A viruses are zoonotic in nature. Influenza A viruses have caused five pandemics, based on virus isolation and nucleotide sequencing, during the last 150 years. The World Health Organization surveillance network for influenza was formed about five decades back, which has developed into the WHO Global Influenza Programme. The role of WHO and Recommendations for National Measures to be taken before and during pandemics has been outlined in the ‘WHO Global Influenza Preparedness Plan’. The World Health Organization had on 11 June 2009, declared the swine flu ‘H1N1’ as a pandemic in humans by raising the pandemic phase to six.

The genome of influenza viruses is a segmented single-stranded RNA of negative polarity. There are eight RNA segments in influenza A and B viruses while influenza C virus has seven RNA segments. The matrix protein and nucleoprotein determine the type specificity. The subtype specificity of influenza A viruses is present in haemagglutinin and neuraminidase proteins. At present there are 18 haemagglutinin subtypes (H1 to H18) and 11 neuraminidase subtypes (N1 to N11). The H17N10 and H18N11 were reported only about a year ago and so far have been isolated only from bats. The rate of evolution in influenza A viruses is the fastest and the slowest in influenza C viruses. The concept of multilevel ‘Virus Clade’ system for the unified system of nomenclature for H5N1 virus strains has produced a lot of clarity about these viruses and will help in their epidemiology and control. The receptor specificity, antigenic drift, antigenic shift, recombination, host species jumping and mixing vessels can affect the evolution of influenza viruses and may have a direct effect on their virulence, pathogenicity, immunity, drug resistance, etc. Although avian influenza is primarily a disease of domesticated poultry, sub-clinical infections do occur in a wide range of feral migratory birds, which play an important role in the spread of the disease. The knowledge of the role of migratory birds, ‘bridge species’ and various ‘reservoirs’ in the epidemiology of influenza continues to grow. Many bird species act as a link/bridge between wild birds in natural habitats and domestic poultry and may have a role in the transmission of AI viruses from poultry to wildlife or vice versa. Therefore, specific surveillance and monitoring of these potential ‘bridge species’ should be carried out in HPAI poultry outbreaks and occurrences of wildlife mortality. Keeping in view the role of migratory birds as potential vectors of HPAI virus subtype H5N1, the ‘International Scientific Task Force on Avian Influenza and Wild Birds’ was established in the year 2005 with many different agencies and scientific bodies as observers and members, with the common aim of giving technical advice and developing early warning systems at the global level. It was thought that the highly HPAI viruses, which cause high mortality in some domestic poultry, do not have a wild bird reservoir, but emerge in domestic poultry populations through mutations in the LPAI viruses perpetuated in wild water birds. However, the observations of rapid spread of H5N1 HPAI virus in 2005–2006, with concurrent outbreaks reported in both domestic and wild birds in different geographical regions have raised concerns and the need to revisit and rethink about the potential role of migratory birds in the epidemiology of the HPAI infection. Migratory birds may transport HPAI H5N1 over long distances as some infected, virus shedding wild birds showing no symptoms or only mild symptoms, have been observed. Therefore, monitoring the role of these migratory birds, wild life and bridge species in the epidemiology should be strengthened.