In this thesis are described the viruses and diseases they cause in fish. Viruses are infectious entities at the edge of the life. In one liter of sea surface water there are 10 billion microorganisms and 100 billion viruses. All groups of living organisms, from unicellular to multicellular are susceptible to viral infections. Most viruses are useful in ecosystem, but certain species are pathogenic, which can have a negative impact on the breeding of important marine organisms. Fish are exposed to stress in the natural environment and in fish farming. When the disease is present in one of the fish population, all fish are not affected immediately. There are many factors that affect the response of potential pathogenic viruses. The pathogen must cause the disease and the fish must be in a sensitive condition for disease. Fish that are controlled in cultivation are more susceptible to virus infections than others in the wild nature. Symptoms of viral fish diseases in controlled breeding can be more easily monitored than in the wild fish. Viral diseases are long-lasting and require monitoring, researching and controlling of infected fish species.