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Infectious diseases in exploited shellfish population cause economical problems world-wide. The characteristics of the disease in the population -i.e. origin, frequency, distribution, development- need to be well known to set up appropriate measures for protection and restoration of shellfish population health. The investigation of disease in a population is the basis of epidemiology. Quantitative investigations include studies, survey, surveillance with specific sampling strategies according to the objectives. European clams (Ruditapes decussatus) and Manila clams (Ruditapes philippinarum), two commercially valuable species, are infected by the protozoan Perkinsus olseni. This pathogen has been associated with epizootic outbreaks involving in heavy mortality of shellfish, including clams. In Europe, the parasite was detected in Italy (the Veneto region), Portugal (Ria Formosa, Ria do Alvor, V.N. Milfontes, Aveiro, Lagoa de Obidos), Spain (Galicia) and France (Chausey Island, Golfe of Morbihan, Marennes-Oleron, Arcachon bay, Leucate and Thau lagoon). We analysed published and unpublished data on Perkinsus olseni infestation in wild and cultured clams collected in Europe, from the English Channel to the Mediterranean coast. Results obtained from these studies clearly demonstrate the detection of Perkinsus olseni in all sampling sites albeit with different intensities. We discuss these disease frequencies according to 1) objective, 2) targeted population, 3) methodological design, 4) quality of diagnostic tests and 5) characterisation of the pathogen. We emphasise the need for a relevant sampling strategy to measure occurrence of the disease.