[Emergent infectious diseases: importance for public health, epidemiology, promoting factors, and prevention].

At the end of the 70s, it was said that eradicating infectious diseases was possible. With the occurrence of AIDS and the discovery of new pathogens the come-back of infectious diseases was noted, 15 to 20 years later, and the concept of emerging infections was defined. An emerging infection is the appearance of a new infection or the increase of its incidence if not new. It also includes infections for which the incidence could increase because of favorable conditions. Several scenarios can be proposed: 1) the appearance of a new infection (AIDS or SARS); 2) the appearance of a known infection where it did not exist previously (West Nile virus infection in the USA); 3) an infection that existed but that was not diagnosed or could not be linked precisely to a particular infectious agent (hepatitis C before the discovery of the hepatitis C virus; 4) a known infection that increases its usual incidence (outbreaks...); 5) infectious diseases for which current conditions may facilitate transmission (decrease of immunization coverage...). The deliberate release of a biological agent is one of the possible scenarios of emergence. Qualitative changes of infectious agents such as the resistance to anti-infectious agents may also contribute, through a selection mechanism, to the emergence of new infectious threat. Emerging infections result from the interaction between the agent, the host, and environment evolving according to human activity. Surveillance, research, laboratory capacity, and an effective public health system are key factors for their control.