Among the infectious diseases observed in bivalve molluscs, viral infections are often not well known because of a certain inadequacy of the diagnostic techniques usually applied during instances of mortality. In fact, photonic microscopy was still the base method for analysing specimens in several laboratories working on pathologies in molluscs. This technique is insufficient in the event of a pathology of viral etiology if it is not supplemented by other approaches such as the electronic microscope, research of cytopathogenic effects on cell lines or detection of the infectious agent with the help of specific reagents (specific antibodies and nucleic probes). Moreover, since no bivalve mollusc cell line is currently available, in vitro research of cytopathogenic effects in a homologous system is not possible. In addition, the absence of cells that produce antibodies in invertebrates means that only direct diagnosis of infections is conceivable. Nevertheless, viruses related to the Herpes viridae family have been described in various species of marine bivalves, linked to abnormal mortality rates of an epizootic or enzootic nature. Although their real pathogenicity has only been rarely demonstrated by experimental reproduction of the disease, these viruses are considered as dangerous agents for shellfish livestock.