ABSTRACT
Intensification of aquacultural practices results in disease outbreaks of prevalent pathogens alongside new and emerging diseases. Water as a medium in an aquatic environment facilitates the spread of disease from infected specimens to naïve fish. The transmission of disease can be either horizontal (fish to fish) or vertical (parent to offspring). Stress is always a predisposing factor to a disease outbreak. The pathogenicity and virulence of the invading pathogen will determine the severity of infection. In addition, the immune status and genetic make-up of fish will define whether the host succumbs to the invading pathogen or not. It is to mention that the immune system of a coldwater fish is generally weak in comparison to a warm water fish given the low temperature which makes fish more susceptible to infectious diseases. Infectious diseases of coldwater fishes are either of bacterial, viral, parasitic, or fungal etiology. In the context of the present chapter, the focus here will only be on viral and fungal pathogens prevalent in cold water aquaculture.
