As aquaculture assumes an expanding role in meeting consumer demands for fishery products, it is natural that they meet safety and quality standards. In addition to residues of agro-chemicals, veterinary drugs and heavy metal organic and inorganic contamination, infectious fish diseases represent one of the most important problems and challenges in aquaculture production, both from an economic and sanitary point of view. Diseases increase cost and reduce profit because of loss in dead fish, cost of treatment or decreased growth rate of diseased and convalescent fish. The fish is usually exposed to pathogens or potential pathogens, but in fish diseases, a simple association between the pathogen and the host fish does not occur. Environmental circumstances that arise in intensive fish farming systems represent a considerable stress, making fish more susceptible to a wide variety of pathogens.