RESUMEN
During recent years the world has experienced the reemergence of old communicable diseases and the emergence of new ones caused by novel pathogens such as the HIV virus and Borrelia burgdorfei. The problem consists mostly in the reemergence of old diseases but specially in industrialized countries new pathogens have also been described. Although the emergence of these infections in rare instances is due to genetic changes of pathogens to more virulent forms, most commonly they are due to changes in the environment and the host. Rapidly deteriorating living standards, desintegration of sanitation and public health infrastructure, cultural changes, migration and variations in behavior are some of the factors involved in the worldwide increase of infectious diseases. The degradation of natural habitats including forests and marine niches accompanied by climatic changes, are also playing an increasing role in the detrimental evolution of these diseases. The global emergence of these diseases calls into question the doctrine of epidemiological transition and directs us to scrutunize the paradigm that bases the prevention of this diseases solely vaccination. The current situation also highlights the limitations of classical epidemiology in dealing with unexpected problems and strongly suggests that this discipline should incorporate into its analysis findings from other fields, including ecological, climatological and economical information. As most of the negative social and economical developments that impinge on the detrimental evolution of these diseases are increasing world-wide, it can be predicted that the problems posed by these infections will continue and perhaps worsen in the near future