Viral diseases have emerged as serious problems to the fish aquaculture industry, as effective biosecurity measures to maintain the health status of fish stocks have increased in the past decades and bacterial diseases have been partially managed. Among rhabdovirus, viral haemorrhagic septicaemia (VHS) disease, is one of the most important viral diseases of salmonid fish in aquaculture. Concerning viral infections, it is particularly evident from studies on VHS Virus in rainbow trout that innate or non-specific defences may play a significant role in resistance to viral diseases. Immune functions carried out by macrophages are thought to be of particular importance in the resistance to viral infections. The protective effect of monoclonal antibodies to the different rhabdoviral proteins was tested in passive immunization experiments. On mammal/virus models leucocyte proliferative responses occur after presentation of a limited number of short viral protein peptides in the membrane of the host infected cells in the major histocompatibility complex context.