Changes in alimentary behaviors can be responsible for the reappearance of parasitic and bacterial diseases. The consumption of raw or undercooked indigenous fishes is, for example, the cause of 30 cases of human diphyllobothriasis reported by the Istituto Cantonale Batteriosierologico of Lugano. In fact, larvae of Diphyllobotrium latum have been found in fishes from sub-alpine and Swiss plateau lakes. The presence of Aeromonas strains in several foods as well as the generalization of the "cold chain" which promotes psycrophylic microorganisms like Yersinia, are factors which contribute to the increase in human-bacteria contacts, allowing bacteria to act as pathogens. For Aeromonas and Yersinia, possible paths of human contamination have been investigated considering their clinical and environmental diffusion by means of biochemical characterization and molecular typing (Ribotyping, Multilocus Enzyme Electrophoresis). The results covered a decade of researches carried out in the South part of Switzerland. Due to the genetic complexity of these two genera, the epidemiology has not been completely understood, but the results show the agreement between their recovery in the environment and the onset in human pathologies.