Co-infections are very common in nature and occur when hosts are infected by two or more different pathogens
either by simultaneous or secondary infections so that two or more infectious agents are active together in the same
host. Co-infections have a fundamental effect and can alter the course and the severity of different fish diseases. However,
co-infection effect has still received limited scrutiny in aquatic animals like fish and available data on this subject
is still scarce. The susceptibility of fish to different pathogens could be changed during mixed infections causing the
appearance of sudden fish outbreaks. In this review, we focus on the synergistic and antagonistic interactions occurring
during co-infections by homologous or heterologous pathogens. We present a concise summary about the present
knowledge regarding co-infections in fish. More research is needed to better understand the immune response
of fish during mixed infections as these could have an important impact on the development of new strategies for
disease control programs and vaccination in fish.