Infl uenza A viruses have 2 main surface glycoproteins -hemagglutinin (HA) and neuraminidase (NA) -which have 16 and 9 subtypes respectively. The infl uenza A virus subtypes are classifi ed on the basis of the different HA and NA glycoprotein subtype combinations. All the subtypes can affect birds which are the natural hosts. Only a few subtypes are capable of infecting humans. The past few years have seen the emergence of two new infl uenza A subtypes which have become a cause of concern for the global community. These are the highly pathogenic H5N1 infl uenza A virus (H5N1) and the pandemic 2009 infl uenza H1N1 virus. The H5N1 virus emerged initially in 1997 and then in 2003. Since 2003 this virus has caused widespread disease and death in poultry. In humans the number of cases infected with this virus is few but the mortality is very high. The second infl uenza virus, the pandemic H1N1 or the swine origin infl uenza A (H1N1) virus emerged this year and within a few months quickly spread throughout the world. The virus acquired the ability for effi cient human-tohuman spread which resulted in a large number of infected individuals. Fortunately the mortality has been low.