Marine viruses were little studied until 1989, when they were discovered to be extremely abundant in the sea. Virology is now a growing field of science in coral reef research, largely related to an increase in the frequency of coral bleaching events and other coral diseases. Because viruses are obligate symbionts, they are generally perceived as parasitic and harmful to their hosts. However, evidence that viruses confer benefits to their hosts is growing and their role as mutualists is emerging. Here we review both the detrimental and beneficial aspects of viral infections and argue that as the field of coral virology expands, in addition to their pathogenicity, the idea that viruses represent functionally beneficial components of the coral holobiont be considered.