The multiplicity of fish and shellfish introductions or translocations was amplified in the last few decades and was accompanied by the introduction of pathogens into inland hydrosystems. However, the analysis of pathogens' origins revealed that some paths remain underestimated. The progress in the methodologies and techniques of identification has lead to recent advances in the analysis of pathogen introductions, but with this progress the problem of perception thresholds arose. A review of the literature reveals that almost one hundred pathogen species were introduced into European hydrosystems. The biological characters in favour of the establishment of the pathogens are described. The diversity and the apparent non predictibility of the introduced pathogens' impacts are detailed both in field and theoretical contexts. A conceptual framework of risk analysis and propositions is provided.