Through the recent examples of diphtheria in the former Soviet Union, plague in India, and trends in pneumonia mortality in the United States, the author, in this article, illustrates issues in emerging infectious diseases. The Centers for Disease Control's plan, Addressing Emerging Infectious Disease Threats: A Prevention Strategy for the United States, is summarized. Initial efforts to implement this plan are described, with particular focus on the development of Emerging Infections Programs, which are conducting epidemiologic and laboratory projects on several infectious diseases, including invasive bacterial diseases, unexplained deaths, foodborne diseases, and ehrlichiosis in four population-based sites in the United States.