Avian influenza, an infectious disease of birds, is caused by type A strain of the influenza virus. The disease, which was first identified in Italy more than 100 years ago, occurs worldwide. Avian influenza viruses are mainly distributed by migratory birds. Various animals like birds, pigs, horses, sea mammals and, finally, humans are susceptible to influenza A viruses. The high possibility of genomic changes like gene shift and drift are caused by the segmented RNA genome.