In this chapter, we examined in detail how biosurveillance systems detect and characterize outbreaks. We described the overall process as comprising three distinct subprocesses— case detection, outbreak detection, and outbreak characterization. Although we described them as separate steps, one triggering the other, we expect that these processes will become more tightly integrated in the future and that the distinctions between these processes will blur.

Case detection is a front-line activity in biosurveillance, which is accomplished by diverse methods, including detection by clinicians, laboratories, screening programs, and, increasingly, computers. Outbreak detection and characterization depend on case detection. Outbreak detection is based on continuous analysis of human and animal data by people working in health departments, the animal healthcare system, and hospital infection control, as well as by astute citizens. Outbreak characterization is an intermittent process that is triggered by outbreak detection. It is the process by which investigators elucidate characteristics of an outbreak that are important for disease control (e.g., causative biological agent, source, and route of transmission). Characterization is based on intensive collection of additional data when an outbreak is suspected or confirmed. Outbreak characterization is the least automated process in biosurveillance at present, but the future role of automation is already recognized (e.g., a recent report from the National Defense University identifies integrated automated event characterization system based on epidemiological, biological, and chemical models and artificial intelligence as a key element in an advanced biosurveillance system (Thompson et al., 2005).

Each of these processes involves many individuals with different skills and many organizations with diverse and sometimes overlapping responsibilities. This situation is unlikely to change anytime in the near future, which is why we consider multiorganizational and multidisciplinary to be fundamental properties of biosurveillance that we must respect when designing biosurveillance systems.

Each of these processes is also data and knowledge intensive (also fundamental properties of biosurveillance). The processes depend not only on substantial data collection but also on mechanisms for the storage, distribution, and presentation of these data. When analyzing these data, people and, increasingly, computers, must bring enormous amounts of knowledge to bear. As in Chapter 1, the analytic processes can perhaps best be summarized by an analogy. The very best outbreak investigators have minds like the great Sherlock Holmes. They are capable of great leaps of insight that appear "elementary'' only in retrospect.They arrive at the scene of an outbreak, assimilate the information available from patients and other observers, collect clues, and generate hypotheses about the culprit (the biological agent) and his accomplices (food, water, mail). Their secret: exhaustive knowledge about the modus operandi of hundreds of biological agents produced by scientific studies of past outbreaks. The very best investigators use this knowledge in a way that the clues that they receive and the evidence they collect (e.g., symptoms, test results, and epidemiological patterns) ultimately lead them to

5 The word forensics is derived from the Latin forensis meaning legal affairs. Forensic attribution assigns responsibility to an individual for an act (but not necessarily to a level required by a specific court as connoted by the term forensic science). Forensic attribution can be difficult as demonstrated by the two bioterrorist events that occurred in the U.S. Investigators did not identify the party responsible for the Salmonella outbreak in The Dulles, Oregon—the Bhagwan Shree Rajneesh cult. Although they considered intentional contamination as the source of the outbreak during their investigation, they rejected this idea because there were other plausible theories and no claims of responsibility, no motive, and no observed unusual behavior. Ultimately, an unrelated criminal investigation of the Bhagwan Shree Rajneesh cult uncovered the fact that the cult was responsible for the Salmonella outbreak. The party or parties responsible for the 2001 anthrax letters were never identified.

the biological agent and its source. The deductive techniques they use are numerous, and the selection is dictated by the problem at hand. The potential for formalizing and encoding this knowledge in computer-supported biosurveillance systems is significant.

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