Viruses are the most abundant members of marine ecosystems and play an enormous role in ocean processes through their interactions with all types of marine organisms. This short review provides examples of the dramatic increase in our knowledge of the diversity of marine viruses as pathogens of bacteria, protists, molluscs, crustaceans, cnidaria, reptiles, fish and mammals. Several examples are provided showing evidence of evolution of new strains, changes in virulence, and transfer of viruses between ecosystems. The natural and anthropogenic causes of these shifts are discussed. Despite considerable advances in recent years, knowledge of the importance of viruses in many important groups of marine organisms is lacking or incomplete. Suggestions for future investigations necessary to understand the dynamics of biogeochemical processes and the impacts of disease in our oceans are proposed.