drome, fibromyalgia, and irritable bowel syndrome, to name a few. Some of these conditions are, in fact, not genuinely “new:” they carry new labels for the same or a similar constellation of symptoms characterized, in some cases, up to a century ago.2

The recognition of a new disease is far from straightforward (Wegman et al., 1997). The simplest statement is that it is a process (Kety, 1974), often taking years. The purpose of the process is to demonstrate that patients are affected by a unique clinical entity distinct from all other established clinical diagnoses. The individual “steps” for gathering and interpreting evidence are not clear-cut. Evidence from biomedical research plays a prominent, but not necessarily exclusive, role in defining and classifying a new disease. Social factors, including culture and economics, influence the recognition, classification, and definition of a new disease (Rosenberg, 1988; Aronowitz, 1998; Wessely et al., 1998). The relative roles of scientific and social factors most likely vary from disease to disease, and era to era. The arbiters of what is a disease (and what is not) are health professionals, organized through public or professional organizations (Wegman et al., 1997), yet without explicit rules for decisionmaking, as explained later.

The process of disease recognition, in its most general features, is captured in Figure D.1. It must be understood at the outset that this is neither a linear nor a carefully articulated process. The process begins with detection of patients whose symptoms cannot be explained readily by existing diagnoses, hence the widely used term “unexplained” symptoms or illness. “Illness” is the term used to refer to patients’ subjective experience of morbidity that they report to their clinicians, in contradistinction to what clinicians diagnose (Eisenberg, 1977). Clinicians or epidemiologists then look for patterns or clusters of symptoms that occur together in the same patient and across many patients. When patterns of symptoms are detected, experts formulate a working “case definition” that establishes classification criteria for a potentially new syndrome. A case definition typically contains a mix of clinical, laboratory, and/or epidemiologic criteria.

The development of the first case definition is a vital milestone designed to spur research and surveillance. More like a hypothesis than a conclusion, the first case definition is an early step in the process of identifying a new clinical entity. Case definitions are protean, frequently changing over time as new evidence comes to light. They are not designed for making diagnoses, which is considered the province of medical practice (Wharton et al., 1990).

A case definition seeks to formulate criteria that effectively identify and distinguish a new patient population from patient populations with recognized


What is now diagnosed as chronic fatigue syndrome is strikingly similar to the nineteenth century descriptions of the condition labeled neurasthenia (Wessely et al., 1998). Lyme disease is an infectious disease that has been traced back to a disease characterized at the beginning of the twentieth century (Aronowitz, 1991).

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