In marine molluscan aqauculture, infectious agents are regularly cited in literature associated with mass mortalities as emergin diseases, but the causal link is often based on very few argumeents. 24 ways to demonstrate causal links from epidemiolgical, ecological and marine pathology literature feasible for marine molluscs are given. Those criteria were applied to the ewample of Bonamia ostreae for Ostrea edulis and Herpesvirus for spats and larvae of Crassostrea gigas. Three principles could be applied in marine field for choosing criteria wich should be taken in priority : the demonstration must be given at three levels of organization, organ individual, population, the demonstration must keeps maximum epidemiological criterias, and then mixes experimental and field work. Results shown that the demonstration is done for Bonamia ostreae but still difficult and not completely done for Herpesvirus on spats.