With over 2200 species, myxozoans are microscopic obligate spore-forming parasites affecting marine and freshwater fish worldwide. They have complex life cycles requiring alternate vertebrate (mostly fish) and invertebrate (mostly worms) hosts. Myxozoans can cause emerging diseases in many commercial fish species, including tilapia, catfish, carp, trout and salmonids, inflicting economic and environmental damage. Myxozoans belong to the phylum Cnidaria, alongside free-living corals, sea anemones and jellyfish. Although they are highly reduced compared to free-living cnidarians, myxozoans have retained the phylum-defining stinging organelles, known as nematocysts, which are essential for initiating fish infection. To explore the adaptation of myxozoan nematocysts to parasitic life and their role in host infection, we studied their structure, firing process and protein content. High-speed imaging revealed that activation of myxozoan nematocyst results in anchoring to the host body and injection of parasite content. To understand fish-parasite interaction, we used the newly discovered myxozoan Myxobolus bejeranoi, which infects gills of hybrid tilapia at more than 80% prevalence, leading to high mortality rates. We characterized the temporal progression of M. bejeranoi infection and the immune response of the host fish by performing transcriptomic analyses of both the sporulation site and the immune organs. We found that the parasite employs an immune evasion strategy of a thorough shutdown of the immune response, including deactivation of secreted cytokines and of a transcription factor responsible for T helper cell differentiation. The consequence is an immune-deprived fish, which is expected to be highly susceptible to other opportunistic pathogens. These findings suggest that M. bejeranoi is a highly efficient parasite that can disable the defense mechanisms of its fish host. Lastly, I will discuss the specificity of myxozoans as well as their global distribution, including concerns about fish stock import and the efficiency of health testing of imported livestock.