This project examined whether microplastic ingestion modulates disease expression in a Pacific Northwest (PNW) salmonid species (rainbow trout). The physical presence of microplastics in some marine organisms is known to elicit an immune response. In situations where an organism is exposed to and ingests microplastics, the immune system may be compromised and thus be less capable of dealing with additional stressors, namely a virus common to salmonid species particularly where they exist in close proximity in large numbers (i.e., aquaculture facility). The virus in question is Infectious Hematopoietic Necrosis Virus (IHNV), one of the primary causes of infectious disease loss of salmonids in the PNW. Rainbow trout were reared from eggs in a controlled system to the size of interest. Kidney immune cells from these fish were assayed with weathered and virgin microplastics to observe the humoral immune system response and potential phagocytosis using 3 size ranges of polystyrene and nylon because these are the likely polymers encountered in the aquaculture facility (from buoys and netting). A number of biomarkers were examined, the presence of which would indicate an immune response has been triggered (i.e., expansion of innate cells, developing B cells, and plasmoblasts). The project then measured fish mortality in response to various microplastics concentrations, microplastics coincident with IHNV, and a control exposed to natural fibers (Spartina alterniflora). Mortality was tracked to generate LD50. Fish were tested to confirm death was due to the virus and in vivo humoral immune response markers were examined. In addition, the pathways by which microplastics gain entry into the internal tissues were determined (= mechanism of toxicity) by examining the gills, the gastrointestinal tract and other blood filtering organs for microplastics accumulation and checking for concentration of relevant chemical additives. This interdisciplinary work will make significant contributions to our understanding of the nexus of microplastics, immunology and fish disease.