In the late 1970s, there was talk of "the end of infectious diseases." With the emergence of AIDS and the discovery of new infectious agents, the "return of infectious diseases" was declared 15 to 20 years later, giving rise to the concept of "emerging infections." "Emergence" refers to something that "exceeds the average level, attracts attention, or stands out from the crowd..." In epidemiological terms, it refers to a disease that appears or whose incidence increases in a given location. Infections whose incidence could increase due to conditions conducive to their transmission must also be included. Different scenarios can be proposed depending on whether the disease appears de novo (AIDS or SARS); appears where it did not previously exist (West Nile virus in the United States); existed without being able to be accurately diagnosed or linked to a specific infectious agent (hepatitis C before the discovery of HCV); deviates more or less rapidly from usual fluctuations (epidemic); or is caused by an agent whose transmission conditions are conducive to its resurgence (decline in vaccination coverage, etc.). Bioterrorism is one such emergence scenario. Qualitative changes in infectious agents (resistance to anti-infectives, etc.) can, through a selection mechanism, lead to emergences. Infectious outbreaks result from the interaction between the agent, the host, and the environment, which evolves with human activity. The fight against emerging infections is based on surveillance, high-level microbiological expertise, and the response capacity of the public health system.