Aquatic Microbiology is dedicated to the advancement of microbial research in aqueous environments, with an emphasis on freshwater, estuarine, and oceanic ecosystems. Aquatic microbes have a variety of roles in ecosystems and are essential to the biogeochemical cycles on the planet. Aquatic microorganisms are also genetically, physiologically, and environmentally varied, with a wide range of evolutionary, adaptive, and physiological responses. Organisms' immune systems defend them against parasites, pathogens, and viruses. The majority of what we know about the composition, function, and regulation of the two basic branches of the immune system, innate immunity and adaptive immunity, comes from mouse and human studies. However, there has recently been a surge in interest in fish immunology for a variety of reasons. Fish immune systems serve as useful comparative outgroups for evolutionary biologists studying immune system evolution. Such comparisons should eventually lead to a better understanding of immune system design principles in general. Fish immune systems are well developed, with full representation of all known core components of innate and adaptive immunity, albeit with specific specializations and unique traits. Understanding the structure and function of the immune system in fish is critical for the development of new technologies and products that will boost production.