To become established in new populations, pathogens must amplify and spread within their host species, thereby creating a transmission focus. The conditions necessary for successful implantation depend on the nature of the microorganism and its mode of transmission. The establishment of a tropical infectious disease requires a combination of environmental and human factors, as well as favorable ecological conditions. This complex interplay, in which humans have an increasingly influential role, makes it difficult to predict which pathogens are most at risk. Despite the increasing frequency of introductions, few new pathogens become established in Europe. We must prepare for these unpredictable events by perfecting clinical, microbiological and entomological surveillance networks and rapid detection systems.