In 1965, the Director General of the World Health Organization (WHO) established the epidemiological surveillance unit in WHO's Division of Communicable Diseases. In 1968, the 21st World Health Assembly affirmed the three main features of surveillance: a) the systematic collection of pertinent data, b) the orderly consolidation and evaluation of these data, and c) the prompt dissemination of results to those who need to know-particularly those in position to take action . In addition, "epidemiologic surveillance" was said to imply "...the responsibility of following up to see that effective action has been taken." This addition emphasises the cyclical nature of the surveillance process, as outlined in figure 1.
The classical model of surveillance thus includes three major processes:
This is often shown as a cyclical process, with a fourth process of public health response (intervention), which may result in changes that will then be evaluated by the collection, analysis and interpretation of data. This surveillance cycle is depicted in figure 1.
1. National and global surveillance of communicable disease. Report of the technical discussion at the Twenty-First World Health Assembly. A21/Technical Discussions/5. Geneva, Switzerland: World Health Organization, May 1968.