[The challenge of emergent diseases and the return to descriptive epidemiology].

The definition of emergent and re-emergent diseases, patterns of occurrence during the last 25 years, and the determinants of this occurrence are presented. The importance of descriptive epidemiology and its use in the investigation of these health problems, especially in view of epidemiologists' tendency to give less attention to it, finally, is emphasized the challenges that the emergent diseases pose for public health practice and for the methodological development of descriptive epidemiology are also set out. These challenges are considered in three fields: biosecurity, surveillance systems and descriptive epidemiology techniques.