CHAPTER ONE INTRODUCTION AND LITERATURE REVIEW 1.1 Introduction Newcastle Disease (ND) is the most important viral disease of poultry in the world including developing countries (Adene, 1999; Spradbrow, 1997). In Africa and Asia ND is a major constraint against the development of both industrial and village poultry production (Alders et al., 2001). ND has been reported as one of the greatest constraints to the development of rural poultry production in Nigeria (Shamaki et al., 1989; Oladele et al., 2003). ND is a highly contagious and commonly fatal ribonucleic acid virus infection of birds which can cause up 100 percent mortality in susceptible chickens. Many avian species may become infected but dramatic losses are seen most often in domestic fowl and to a lesser extent in turkeys and Pheasants (Rosenbeger, 1981; Vickers and Hanson, 1982; Gordon and Jordan,1982) ND is caused by avian paramyxovirus type-1(APMV-1) which is classified with other paramyxoviruses in the genus Avulavirus, subfamily paramyxovirinae, family paramyxoviridae, and order mononegavirales. It is an enveloped virus and has a negative sense single strand RNA genome.