estimate the magnitude of the problem, describe the natural history of the disease, identify factors responsible for emergence, facilitate laboratory and epidemiological research, and assess the success of specific intervention efforts.

Unfortunately, there is insufficient awareness of and appreciation for the value of comprehensive surveillance programs. Even among public health personnel, involvement in surveillance activities is often limited to collecting and transmitting disease-related data, a viewpoint that can mask the objectives and significance of the overall effort. Some health care and public health professionals are unfamiliar with surveillance methods, mainly because the topic is covered inadequately in medical schools and even in schools of public health (Thacker and Berkelman, 1988). The result is incomplete, underrepresentative, and untimely disease reporting. Poor surveillance leaves policymakers and practitioners without a basis for developing and implementing policies for controlling the spread of infectious diseases.

Surveillance can take many forms, from complex international networks involving sophisticated laboratory and epidemiological investigations, to small, community-based programs or a single astute clinician. Disease surveillance often is a passive process that is based on individual health care workers who report instances of unusual or particularly contagious human illnesses, usually to a government health agency. In other instances, more formal surveillance can take place, in which public health workers actively seek out cases of disease and report their findings regularly to a central data collection point.

The importance of surveillance to the detection and control of emerging microbial threats cannot be overemphasized. Active monitoring of such factors as population growth and migration, vector abundance, development projects that disturb the environment, and natural environmental factors (especially temperature and precipitation) is an essential component of surveillance and can influence the spread of emerging infectious diseases and the effectiveness of efforts to control them.

Surveillance is important to any disease control effort; it is absolutely essential if that effort's goal is eradication. Without the information obtained through disease surveillance, it is not possible to know how and where disease control efforts should be focused or to analyze the impact of ongoing efforts. The smallpox eradication program, discussed below, is an excellent example of the use of surveillance for case finding and program monitoring.

An often overlooked but very significant contributor to the success of global smallpox eradication was disease surveillance. Of course, smallpox eradication would have been impossible had there not been an effective