Infectious bursal disease (IBD) is an extremely contagious infection affecting young chickens. Till 1987, the infectious bursal disease virus (IBDV) was characterized by low virulence. Very virulent IBDV strain, which may cause mortality rates of 90%, was first isolated in the Netherlands in 1988. Then it started to spread to the UK, Japan, Belgium, and most parts of the world. Infectious bursal disease virus IBDV has 2 routes of infection; oral and intra-ocular routes. The virus is classified into 4 pathogenic strains which are classical virulent strain, attenuated strain, antigenic variant...