Pathogens often make host jumps and subsequently evolve in new systems, as exemplified by the COVID-19 pandemic. A long standing question is will pathogens evolve increased or decreased virulence (host morbidity and mortality) as they adapt to new hosts. Answering this question is an important first step in estimating the threat emergent pathogens pose, and adequately directing resources to manage them. Aquaculture is a particularly unique environment for emergent pathogens , with various aquaculture practices predicted to drive virulence evolution in different directions. However, most of these predictions are based solely on theoretical work, with limited experimental studies. We investigated the virulence evolution trajectory of infectious hematopoietic necrosis virus (IHNV) after it made a host jump into rainbow trout (Oncorhynchus mykiss) aquaculture from sockeye salmon (Oncorhynchus nerka ) in the 1960-70’s. We used a collection of 1000’s of viral isolates spanning 50 years from the time of the host jump to present day, and in vivo common garden experiments in both the novel and ancestral host, to assess the change in viral isolate virulence through time. We found that IHNV dramatically increased virulence in the novel host (rainbow trout) at the time of the host jump and has continued to gradually increase virulence through time (Fig. 1) . An initial drop in virulence was also observed in the ancestral host (sockeye salmon) at the time of the host jump, but evidence of subsequent virulence evolution in the ancestral was not observed. Our findings suggest that emergent pathogens may evolve increased virulence in aquaculture. We discuss how various aquaculture practices might drive virulence evolution and offer possible management solutions.