Nervous necrosis virus(NNV)can cause mass mortalities of many marine and freshwater fishes in the world.The genome of NNV consists of two segments,positive sense,single strand RNAs,named RNA1 and RNA2.A sub-genomic RNA3 is synthesized during virus replication.RNA1 encodes RNA-dependent RNA polymerase;RNA2 encodes capsid protein,the only viral structural protein,while RNA3 encodes B1 and B2 proteins,which are non-structural viral proteins.Based on the gene of the capsid protein,NNV are classified into four genotypes,named striped jack nervous necrosis virus(SJNNV),tiger puffer nervous necrosis virus(TPNNV),barfin flounder nervous necrosis virus(BFNNV)as well as red spotted grouper nervous necrosis virus(RGNNV).However,there are only A,B and C three serotypes of the virus,in which SJNNV belongs to A serotype,B belongs to TPNNV,BFNNV and RGNNV belong to C serotype.There are vertical and horizontal transmission pathways of the virus and it is widely distributed in wild-ranged and cultured fish.Blocking the transmission of the virus between wild and cultured fish as well as the development of novel vaccine will be the trend of the virus research in the future.