Infections in fish leading to disease outbreaks are a major concern for the aquaculture sector because they can result in significant economic damage owing to morbidity and death. The high fish-rearing densities currently used in aquaculture enable the transfer and spread of pathogenic microorganisms and are often a primary cause of such catastrophic outbreaks [4]. Intensive farming practices exert huge stresses on cultured aquatic species, compromising their innate immune defenses against various disease-causing bacterial and viral pathogens. Adequate husbandry and overall management, including biosecurity, nutrition genetics, system management, and water quality, are crucial for aquaculture production in all intensive culture farming practices, irrespective of whether individual or several species of fish are produced in dense populations [8]. In China, India, and Vietnam, fish diseases are estimated to contribute to more than 30% of the overall production loss [9]. Several bacterial and viral pathogens and parasites are opportunistic and occur in the environment or as asymptomatic carriers on some fish, which renders aquaculture facilities highly susceptible to disease outbreaks and hinders the development of an efficient, cost-effective, and stable aquaculture process [10]. The appearance and progression of fish disease are determined by the relationship between the pathogen, host, and environment. Stressful conditions, including high population density, change in temperature, and hypoxia, can hasten the spread of pathogenic bacteria and result in major disease outbreaks [11]. Thus, multidisciplinary studies on the characteristics of potential fish pathogens, the biology of the fish hosts, and an adequate understanding of the global environmental factors affecting are important to investigate appropriate measures for the prevention and control of the major diseases limiting fish production in aquaculture.

2. Bacterial Pathogens of Fish