Technical Abstract: Abstract. The impact of infectious diseases remains one of the limiting factors to successful economic production of fish. Infections by bacteria, viruses, fungi, parasites, and rickettsia-like organisms are the cause of diseases that are characterized as acute, chronic, or asymptomatic in fish. The occurrence of disease is an interaction between the health status of the fish and infectious microbes present in the fish and the environment. There is an increasing awareness that stressful environments, husbandry practices, pollutions, social stresses, infection by other microbes, or all of these factors, suppress the immune system of healthy fish. The authors propose a bi-directional disease continuum-model to help explain the weakening of the immune system against microbes as the result of neuroimmune changes caused by environmental, husbandry and social stresses, and sublethal exposure to infection or neuro-immunotoxins.