Emerging infectious diseases pose a growing health threat to people in this country and around the world. The causes of this increase are complex and often difficult to anticipate. For example, increased development, deforestation, and other environmental changes have brought people into contact with animals or insects that harbor diseases only rarely encountered before. Not all emerging infections are unfamiliar diseases, however. Some pathogens have developed resistance to the antibiotics that brought them under control just a generation ago. Moreover, the threefold increase in international travel during the past 20 years and greater importation of fresh foods across national borders allow infectious diseases to spread rapidly. As these diseases travel, they interact with growing numbers of people who have weakened immunity, such as transplant recipients, elderly persons, patients treated with radiation, and those infected with HIV/AIDS. With the introduction of antibiotics in the 1940s and the development of vaccines for diseases like polio, there was widespread optimism that infectious diseases could be eliminated completely. As a result, public health officials shifted some monitoring efforts to other health problems, such as chronic diseases. By 1986, CDC had discontinued surveillance of drug-resistance trends in tuberculosis. The resurgence of tuberculosis and the appearance of HIV/AIDS thus caught the nation’s public health system off guard. Today, infectious diseases account for considerable health care costs and lost productivity. In the United States, an estimated one-fourth of all doctor visits are for infectious diseases. Foodborne illnesses, some of which were unrecognized 20 years ago, are estimated to cause up to 33 million cases and 9,000 deaths annually and to cost as much as $22 billion a year. The number of pathogens resistant to one or more previously effective antibiotics is increasing rapidly, adding to health care costs and threatening to return the nation to the pre-antibiotic era. Antibiotic resistance limits effective treatment options, with potentially fatal results. Resistant infections that people acquire during hospitalizations are estimated to cost as much as $4 billion and cause 19,000 deaths a year. Surveillance is public health officials’ most important tool for detecting and monitoring both existing and emerging infectious diseases. Without an adequate surveillance system, local, state, and federal officials cannot know the true scope of existing health problems and may not recognize new diseases until many people have been affected. They rely on surveillance data to focus their staff and dollar resources on preventing and controlling the diseases that most threaten populations within their jurisdictions. Health officials also use surveillance data to monitor and evaluate the effectiveness of prevention and control programs. Because known diseases can become emerging infections by changing in unanticipated ways, the methods for detecting emerging infections are the same ones used to monitor infectious diseases generally. These methods can be characterized as passive or active. When using passive surveillance methods, public health officials notify laboratory and hospital staff, physicians, and other relevant sources about disease data they should report. These sources in turn must take the initiative to provide data to the health department, where officials analyze and interpret the information as it comes in. Under active surveillance, public health officials contact people directly to gather data. For example, state or local health department staff could call commercial laboratories each week to ask if any tests conducted for cryptosporidiosis yielded positive results. Active surveillance produces more complete information than passive surveillance, but it takes more time and costs more. Infectious diseases surveillance in the United States depends largely on passive methods of collecting disease reports and laboratory test results. Consequently, the surveillance network relies on the participation of health care providers, private laboratories, and state and local health departments across the nation. States have principal responsibility for protecting the public’s health and, therefore, take the lead role in conducting surveillance. Each state decides for itself which diseases will be reported to its health department, where reports should be submitted, and which information it will then pass on to CDC. The surveillance process usually begins when a person with a reportable disease seeks care. To help determine the cause of the patient’s illness, a physician may rely on a laboratory test, which could be performed in the physician’s own office, a hospital, an independent clinical laboratory, or a public health laboratory. State and local health departments that provide clinical services also generate laboratory test results for infectious diseases surveillance. Local health departments are often the first to receive the reports of infectious diseases generated by physicians, hospitals, and others. Health department staff collect these reports, check them for completeness, contact health care professionals to obtain missing information or clarify unclear responses, and forward them to state health agencies. Staff resources devoted to disease reporting vary with the overall size and mission of the health department. Since nearly half of local health agencies have jurisdiction over a population of fewer than 25,000, many cannot support a large, specialized staff to work on disease reporting. In state health departments, epidemiologists analyze data collected through the disease reporting network, decide when and how to supplement passive reporting with active surveillance methods, conduct outbreak and other disease investigations, and design and evaluate disease prevention and control efforts. They also transmit state data to CDC, providing routine reporting on selected diseases. Many state epidemiologists and laboratory directors provide the medical community with information obtained through surveillance, such as rates of disease incidence and prevailing patterns of antimicrobial resistance. Federal participation in the infectious diseases surveillance network focuses on CDC activities—particularly those of the National Center for Infectious Diseases (NCID), which operates CDC’s infectious diseases laboratories. CDC analyzes the data furnished by states to (1) monitor national health trends, (2) formulate and implement prevention strategies, and (3) evaluate state and federal disease prevention efforts. CDC routinely provides public health officials, medical personnel, and others information on disease trends and analyses of outbreaks. Through NCID and other units—such as the National Immunization Program and the National Center for HIV, Sexually Transmitted Diseases, and Tuberculosis Prevention (NCHSTP)—CDC offers an array of scientific and financial support for state infectious diseases surveillance, prevention, and control programs. NCID officials said that most of their 1,100 staff and $186 million budget in fiscal year 1998 were devoted to assisting state infectious diseases efforts. For example, CDC provides testing services and consultation not available at the state level; training on infectious diseases and laboratory topics, such as testing methods and outbreak investigations; and grants to help states conduct diseases surveillance.The Epidemiology Program Office provides training and technical assistance related to software for disease reporting and oversees data integration efforts. Public health and private laboratories are a vital part of the surveillance network because only laboratory results can definitively identify pathogens. In addition, they often are an essential complement to a physician’s clinical impressions. According to public health officials, the nation’s 158,000 laboratories are consistent sources of passively reported information for infectious diseases surveillance. Independent commercial and hospital laboratories may also share with public health agencies information gathered through their private surveillance efforts, such as studies of patterns of antibiotic resistance or the spread of diseases within a hospital. Every state has at least one state public health laboratory to support its infectious diseases surveillance activities and other public health programs. Some states operate one or more regional laboratories to serve different parts of the state. In five states—Iowa, Nebraska, Nevada, Ohio, and Wisconsin—academic institutions, such as university medical schools, provide public health laboratory testing. State laboratories conduct testing for routine surveillance or as part of special clinical or epidemiologic studies. These laboratories provide diagnostic tests for rare or unusual pathogens that are not always available in commercial laboratories or tests for more common pathogens that use new technology still needing controlled evaluation. State public health laboratories provide specialized testing for low-incidence, high-risk diseases, such as tuberculosis and botulism. Testing they provide during an outbreak contributes greatly to tracing the spread of the outbreak, identifying the source, and developing appropriate control measures. Epidemiologists rely on state public health laboratories to document trends and identify events that may indicate an emerging problem. Many state laboratories also provide licensing and quality assurance oversight of commercial laboratories. State public health laboratories are increasingly able to use new advanced molecular technology to identify pathogens at the molecular level. Often, these tests provide information that is used not to diagnose and treat individual patients but to tell epidemiologists whether cases of illness are caused by the same strain of pathogen—information that is not available from clinical records or other conventional epidemiologic methods. Public health officials have already used this type of laboratory information to identify the movement of diseases through a community in ways that would not have been possible 5 years ago. For example, staff in Minnesota’s laboratory use a molecular technology called pulsed field gel electrophoresis (PFGE) to test “isolates” (isolated quantities of a pathogen) of E. coli O157:H7 that laboratories in the state must submit. From 1994 to 1995, the resulting DNA fingerprint patterns identified 10 outbreaks—almost half of which would not have been identified by traditional surveillance methods. Using the laboratory results, epidemiologists were able to find the sources of contamination and eliminate them, thus preventing additional infections. CDC laboratories provide highly specialized tests not always available in state public health or commercial laboratories and assist states with testing during outbreaks. The staff at CDC’s laboratories also have a broad range of expertise identifying pathogens. These laboratories help diagnose life-threatening, unusual, or exotic infectious diseases; provide information on cases of infectious diseases for which satisfactory tests are not widely or commercially available; and confirm public or private laboratory test results that were atypical or difficult to interpret. According to NCID officials, CDC laboratories provide testing services and consultations on conducting tests or interpreting results to every state. CDC also conducts research to develop improved diagnostic methods and trains state laboratory staff to use them. While state surveillance and laboratory testing programs are extensive, not all include every significant emerging infectious disease, leaving gaps in the nation’s surveillance network. Each state decides which diseases it includes in its surveillance program and which diseases it routinely reports to CDC. Many state epidemiologists believe their surveillance programs need to add or focus more attention on important infectious diseases, including hepatitis C and antibiotic-resistant diseases.