The multiplicity of circumstances by which a host can capture a microbiological agent can be broken down into two simplified processes: 1. encounter between host and microbe, 2. compatibility of two organisms. Given that rodents include species of generally small size, inhabiting continental areas in closer proximity to man than many other mammals, they offer numerous chain of contamination. Microbiological surveillance must both consider the emergence of old and new microbial types and identify ecological and demographic condition placing man at risk. Since it is materially impossible to survey all existing pathogens, priorities must be set according to empirical, epidemiological and ecological considerations. One of the major challenges is how to make microbiological surveillance more effective by way of new tools for detecting demographic changes in rodent populations.