The extraordinary progress in the knowledge of infectious disease, the discovery of antibiotics and effective vaccines are among the great achievements of the nineteenth and twentieth centuries. These achievements have led to a dramatic reduction in the levels of mortality from these diseases. However some factors may suggest that new infective agents may develop, that those diseases that already exist may become more aggressive, and that those diseases that had been practically conquered, may re-emerge in the future. Such factors include: antibiotic resistance, national and international migration, population increase and ageing, increase in the number of immuno-suppressed patients.