Dramatic increases in the volume and speed of international travel and trade in recent years have increased opportunities for diseases to spread across international boundaries. The global reach of the ongoing HIV/AIDS pandemic and the recent appearance in the United States of West Nile virus—a pathogen never before identified in the Western Hemisphere— demonstrate this point. Diseases once regarded as declining in significance have also reemerged in recent years to once again become major global health threats. For example, according to WHO, global reports of yellow fever have dramatically increased over the last 2 decades. The emergence of previously unknown diseases and the development of disease strains resistant to antimicrobial drugs further complicate international disease control efforts. Over the past 3 decades, more than 30 previously unknown diseases have been identified. Many, including Ebola hemorrhagic fever, Rift Valley fever, and Lyme disease, appear to have become threats to human health because of increased human movement into or alteration of the habitats of disease-carrying insects and animals. Excessive, uncontrolled use of antimicrobial drugs has contributed to the evolution of disease strains that are highly resistant to available medications. Infectious diseases can be a substantial obstacle to economic and social advancement in developing countries, where the great majority of cases of such diseases occur. For example, WHO has concluded that Africa’s gross domestic product would be nearly one-third higher than it is today if malaria alone had been eliminated 35 years ago. Development experts believe that the HIV/AIDS pandemic will have a similar impact on African economies. Surveillance provides information for action against infectious disease threats. Basic surveillance functions include detecting and reporting cases of disease, analyzing and confirming this information to identify outbreaks and clarify longer-term trends, and applying the information to inform public health decisionmaking. When effective, surveillance can facilitate (1) timely action to control outbreaks, (2) informed allocation of resources to meet changing disease conditions, and (3) adjustment of disease control programs to make them more effective. According to CDC, factors that can be taken into account in evaluating surveillance systems include their ease of operation; the extent to which health care providers and laboratory personnel actually provide the system with information; and the system’s ability to identify cases of disease, accurately diagnose them, and generate timely and accurate information on disease events and trends. Basic responsibility for disease surveillance and response lies with individual countries. The legal underpinnings for cooperation among countries to control infectious diseases are limited in scope. The primary function of the International Health Regulations—the most important and only binding international agreement on disease control—is to delineate measures that countries may take to protect themselves against epidemics of three diseases: cholera, plague, and yellow fever. To provide national authorities with a basis for applying protective measures, the regulations require countries that record cases of these three diseases to report to WHO, which then makes that information available to other countries. The Regulations do not provide an international framework for addressing threatening epidemics at their source—within countries. At the global level, surveillance functions are carried out through a loose framework that links elements of national health care systems with various entities, including media channels, nongovernmental organizations active in health, and laboratories and other institutions participating in networks focusing on particular diseases and/or regions. Figure 1 presents one illustration of this global “network of networks.” The groupings presented in this figure are not mutually exclusive. For example, national public health authorities may operate WHO Collaborating Centers, participate in epidemiology training networks, and maintain Internet discussion sites. Note 1: UNHCR represents the United Nations High Commissioner for Refugees. Note 2: UNICEF represents the United Nations Children’s Fund. WHO plays a central role in the surveillance framework by working to strengthen national and international surveillance capacity and coordinating international efforts to monitor disease trends, detect and respond to outbreaks, and carry out disease control programs. Foreign assistance agencies such as the World Bank and USAID, as well as private foundations, are important sources of support for strengthening surveillance operations, particularly those taking place in developing countries. For example, in commenting on a draft of this report, the World Bank noted that it is actively working with a number of developing country governments to strengthen their national surveillance systems, within the context of the Bank’s overall emphasis on health. While many technical agencies contribute to framework operations, CDC is the single largest source of expertise and resources available to the international surveillance and response system. The Department of Defense also contributes to global surveillance through its Global Emerging Infections Surveillance and Response System. In commenting on a draft of this report, for example, the department cited its contributions to global surveillance for drug-resistant malaria and influenza. The global surveillance framework’s capacity for serving the public interest varies according to the level of commitment that the international community has made to controlling individual diseases or groups of diseases. The most significant influence on the framework’s development has been the international public health community’s focus on controlling specific diseases. In some circumstances—when a disease can be eradicated with comparative ease or when it poses a high risk of a global pandemic—these programs have attracted broad support and substantial funding. In such situations, public health officials have been able to establish specific goals and create comparatively high-performing systems—including surveillance systems—to support achievement of those goals. Surveillance for other diseases is more limited. The strongest influence on the evolution of the existing surveillance framework has been the collaboration among medical professionals, national governments, and foreign assistance agencies to develop control programs and associated surveillance efforts that focus on specific diseases or groups of diseases. The longest standing of these disease- specific efforts is the global influenza program, which was launched prior to WHO’s founding in 1948. Later, the success of the global effort to eradicate smallpox (1966 through 1977) spurred the creation of other programs designed to eradicate or eliminate global disease threats, such as polio and leprosy, and diseases found in specific regions, such as guinea worm and river blindness, which are both found primarily in Africa.Global consensus in favor of these eradication/elimination campaigns was achieved during the late 1980s and early 1990s, after reduction programs had achieved substantial progress. WHO also collaborates with numerous institutions around the world to maintain programs to control noneradicable infectious diseases such as HIV/AIDS, cholera, tuberculosis, malaria, and dengue. National disease-control programs reflect this focus on specific diseases. They are generally managed through separate programs aimed at specific diseases, such as polio and tuberculosis, or groups of diseases, such as those covered by the Expanded Program on Immunization. Variation in the quality of global surveillance systems can be attributed in large measure to disease characteristics. Under certain circumstances— for example, if a disease can be eradicated or if it poses a high risk of a global pandemic—disease-specific control programs have attracted broad support and have employed this support to create comparatively effective surveillance systems. Surveillance for other diseases, including emerging infections, has received less international support and is more limited. The best surveillance systems have been established to support international campaigns aimed at eradicating or eliminating certain diseases, including polio and guinea worm, and at protecting the global community against influenza—a disease that has the potential to inflict global pandemics. The international community has been supportive of eradication/elimination campaigns because they promise dramatic results—the removal of targeted diseases as public health threats—after relatively short periods of concentrated effort. However, only diseases with certain characteristics can be eradicated or eliminated. In addition to imposing substantial disease burdens—a trait common to many illnesses— diseases that the global community has targeted for eradication or elimination tend to share other characteristics that have encouraged consensus in favor of concerted action. Although the international community has targeted other diseases for eradication or elimination, polio and guinea worm are discussed below to illustrate the characteristics of eradicable diseases and the comparatively high quality of surveillance systems that are created to support international eradication/elimination campaigns. The polio virus and the guinea worm parasite both require human hosts to complete their reproductive life cycles. Both can be controlled by interrupting their transmission from infected to uninfected individuals. Also, available diagnostic tools and approaches make these diseases relatively easy to identify and differentiate from other illnesses. For example, a small but predictable number of polio victims (less than 1 percent) develop acute flaccid paralysis—a condition in which those infected suddenly lose control of the muscles in their limbs. This makes it possible to readily identify communities where intervention may be required. Guinea worm is easily detected when mature worms emerge from infected people’s bodies. Moreover, these diseases generally can be controlled through application of effective, comparatively inexpensive, and easily applied interventions. Polio, for example, can be prevented through immunization with vaccines that are available to developing countries at very low prices. Guinea worm transmission can be dramatically reduced through education and relatively cheap and simple water filtration systems. These characteristics have allowed disease experts to develop clearly stated, technically feasible, time-limited goals and indicators for measuring progress. Advocates for campaigns against these diseases have been able to obtain political commitment and financial support from countries with these diseases and from public and private sources of foreign assistance. For example, the global polio eradication effort has received financial and/or technical support from the governments of the United States, Japan, Norway, Australia, Canada, Denmark, the United Kingdom, and other industrialized countries; Rotary International and other private organizations; and developing country governments. With major financial resources and support from all concerned governments, these campaigns have developed comparatively high- performing surveillance systems. For example, donors and developing country governments have combined their efforts to create a system of active surveillance for acute flaccid paralysis that can promptly identify potential polio cases. This surveillance system has helped reduce the global incidence of polio by 99 percent since 1988. The surveillance effort is ambitious—most countries employ multiple surveillance officers to conduct active surveillance for cases of acute flaccid paralysis.