Emerging infectious diseases are diseases of infectious origin whose incidence in humans has increased within the past two decades or threatens to increase in the near future. Many factors, or combinations of factors, can contribute to disease emergence. New infectious diseases may emerge from genetic changes in existing organisms; known diseases may spread to new geographic areas and populations; and previously unknown infections may appear in humans living or working in changing ecologic conditions that increase their exposure to insect vectors, animal reservoirs, or environmental sources of novel pathogens. Reemergence may occur because of the development of antimicrobial resistance in existing infections (e.g., gonorrhea, malaria, pneumococcal disease) or breakdowns in public health measures for previously controlled infections (e.g., cholera, tuberculosis [TB], pertussis). For current outbreak bulletins on diseases of concern for international travelers one may call the CDC Travelers’ Health hotline at (404) 332–4559.