Several definitions may help to clarify this discussion. First, an “epidemic” refers to an unusual occurrence of disease or a health-related condition or event (e.g., drug-taking) in a specified place, time, and population; in this case, “unusual” means more than expected for that place, time, and population. Epidemics are defined by their unexpected or unusual character rather than by magnitude; in order to promote early response to threats that might grow exponentially if left unattended, there is a quite low quantitative threshold for declaring that an epidemic might be starting.

Second, early in the epidemic process, as a disease, health-related condition, or event begins to mount within a specified place, time, and population, the term “outbreak” sometimes is used. As a term, “outbreak” is less exact than “epidemic,” and some epidemiologists say “outbreak” where others say “epidemic,” but most would agree that outbreak is what we call an epidemic in its earliest stages, and that many outbreaks do not progress to become epidemics. In many ways, an outbreak is like an epidemic’s embryo, and the embryo may not hatch.

Third, in the context of public health work, “surveillance” refers to the intelligence activities: deliberate efforts to detect unusual occurrence of disease, health-related conditions or events, in a manner that can be distinguished by its practicality, completeness of coverage of local area populations, and timeliness, rather than by its accuracy or scientific validity and precision. If it is to be successful, surveillance must occasionally result in a falsely positive warning—that is, an outbreak that remains in embryonic form and does not become an epidemic. In light of the catastrophes that can occur when disease epidemics are not detected until very late stages, the occasional falsely positive warning is the penalty paid in order to escape warnings about public health disasters of major significance that come too late.

This overview of basic public health concepts may come as a surprise to readers who are accustomed to thinking about drug abuse surveillance in terms of the data collection systems discussed in this chapter. None of these data systems is especially timely, and none has a fine-grained coverage of local areas within the nation. If they were refined to give detailed coverage of all local areas, they would be so costly as to be completely impractical.

Nevertheless, a cursory reading of the background history of these data systems reveals that they were intended to provide surveillance information concerning the drug use of the U.S. population (NHSDA, MTF, DUF/ADAM), as well as overdoses and other hazards associated with drug use (DAWN). The questions raised about the suitability of these data to measure drug use must be held in check when this intention is brought into focus. A surveillance system generally is designed to value

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