In this project we will be investigating molecular-, functional-, pathologicaland evolutionary properties associated with low- or avirulent ISA virus (ISAV) HPR0 genotypes, assumed to be precursors of all virulent ISAV strains that cause classical ISA outbreaks. The main objective through this multidisciplinary approach will be to obtain new information that can aid in forming the necessary basis for evaluating important issues related to HPR0, like the risk of infection and its frequency of transition to virulence.