Knowledge of viral infection in marine mammals, a group severely threatened by human activity, is largely limited to the pathology and epidemiology of few endemic viruses. The recent emergence in marine mammals of high-consequence viruses, such as H5N1 avian influenza and rabies, underscores the importance of understanding the ecology of viral transmission in these species. Metatranscriptomic approaches now enable relatively unbiased characterisation of full viral communities that can reveal eco