Study involving targeting of immune-related genes, and monitoring changes in their expression levels is one possibility how to study responses of host to pathogen infection. In fish, like in other vertebrates, activation of both innate and adaptive immune system plays a crucial role against pathogens including viruses which represent a significant threat for fish farming. The purpose of thesis is to: 1) conduct a literature review on economically important viral diseases in fish, 2) summarise information on immune system response of fish to viral infection with the main focus on the expression analyses of immune genes in fish following viral infection, 3) acquire practial experience in basic molecular biology techniques and 4) apply theoretical knowledge in a practical part of thesis, i.e. study the expression patterns of selected immune genes in cyprinid fish.