In the Region of the Americas the emerging and reemerging infectious diseases that had the greatest impact on health, in terms of their incidence and the number of deaths that they caused during the five-year period of 1999-2003, were: malaria, yellow fever, dengue hemorrhagic fever, AIDS, anthrax, and SARS, as well as infection by hantavirus and by West Nile virus. The appearance of epidemics of emerging and reemerging diseases is related to biological, social, and economic factors. Growth in international trade, the movement of large numbers of people across national borders, the variability and genetic adaptability of the causative microorganisms, and inefficiencies in public health systems help to spread infections and epidemics. To avoid or reduce the serious effects of these epidemics, countries should give priority in their national agendas to surveillance of emerging and reemerging diseases and should implement a set of measures to combat the diseases. The most important of these measures is to develop a strategy that is based on early warning and rapid response mechanisms, with personnel and laboratories as well as communications networks that link laboratories with health service providers. This strategy should be backed by priority funding and adequate policies.