Diseases are a major threat to both wild and farmed fish. Pathogen-induced alterations in viability and growth of wild fish stocks can have implications on diversity and ecological status of aquatic ecosystems, as fish are main components of aquatic communities, and they can directly affect the exploitation of wild and farmed fish as a protein source for human consumption. Fish diseases can be a major factor influencing abundance and distribution of wild fish. Disease-related reduction of reproduction and/or age-specific survival can strongly affect wild stocks as it has been demonstrated for several species in distinct geographical locations. The book will be useful for fish farmers, managers, researchers and graduate students interested in fish diseases.