Diseases of fish, caused by biological agents (bacteria, fungi, virus or parasites), are better known in farmed or ornamental fish, since the access to the affected fish coming from the natural environment is less probable. In fact, the fish that are infected by a specific pathogenic agent, natural inhabitants of the marine, estuarine or freshwater ecosystems, are rapidly eliminated from its biotopes by other predatory animals, due to its higher vulnerability and susceptibility to the social interactions.
Diffusion of pathogenic bacteria, fungi, virus or parasites in the aquatic environments is more efficient than in the terrestrial ones. Diagnosis and therapeutics of fish disease have specific difficulties and the application of preventive measures is a very complex issue. Some of those diseases have the same epidemiological problems of the infectious diseases of the terrestrial animals: High spread of diffusion, very significant economic losses, restrictions to fish travel or commerce and some (few) have zoonotic impacts.