In the field of infectious diseases, the French Institute for Public Health Surveillance is responsible for driving and coordinating the anticipation and analysis of infectious disease risks in order to alert health authorities and provide them with the information needed to respond to these risks. In the field of infectious diseases, epidemiological surveillance aims to detect health threats early by identifying warning signals across both human and animal sectors. Since these signals are numerous and vary in quality, they are verified. The risk of a pathogen being introduced and spreading within the country is then assessed through a multidisciplinary review. In this article, we use examples to illustrate various scenarios for detection, risk analysis, and response regarding infectious phenomena present in the intertropical zone and potentially importable into metropolitan France. Whether dealing with agents that are little known because they are rarely identified in mainland France, or with de novo emerging agents, the analysis and response must remain flexible and be adjusted according to epidemiological data and the interaction between the agent, the host, and its environment. These uncertainties will lead us to discuss surveillance needs, with a view to improving responsiveness and expertise. (R.A.)