When Does an Observation Become a Disease?
Words matter.
That may sound like an obvious statement, but nowhere is it more important than in science. Scientific progress depends upon our ability to describe observations accurately, to classify them appropriately, and to communicate those classifications with precision. When terminology faithfully reflects the available evidence, understanding advances. When words begin to imply more than the evidence can support, confusion inevitably follows.
The word disease provides an excellent example.
Most of us think we know what the word means. Yet its history reveals that its meaning has changed considerably over time. The English word disease derives from the Old French desaise, literally meaning lack of ease or discomfort. Its origins are entirely experiential. A person who suffered from a disease simply felt unwell. The word carried no implication regarding pathology, mechanism or cause. It described an experience rather than an explanation.
Modern medicine, however, has transformed the concept. Disease is no longer defined simply by how a patient feels. Instead, it has become an objective description of an abnormal biological state.
Contemporary medical dictionaries differ slightly in their wording, but they consistently emphasise similar themes. Dorland’s Medical Dictionary describes disease as a deviation from or interruption of the normal structure or function of any part, organ or system of the body, manifested by a characteristic set of signs and symptoms. Mosby’s Medical Dictionary, one of the most widely used medical dictionaries in North America and Australasia, defines disease as a condition of abnormal vital function involving any structure, organ or system of an organism, or a specific illness characterised by a recognisable set of signs and symptoms attributable to hereditary, infectious, dietary or environmental factors. Stedman’s Medical Dictionary similarly emphasises interruption or disorder of normal body function, while the Oxford Dictionary of Medicine adds the concepts of recognisable pathology and, where possible, a specific cause. The World Health Organization (WHO) rarely attempts to define disease directly, instead relying upon recognised pathological entities, established diagnostic criteria and clinically reproducible syndromes.
Taken together, these definitions reveal a remarkable degree of agreement. A disease is not simply the presence of an unusual molecule, an unexpected microscopic appearance or an isolated laboratory result. Rather, it represents the convergence of multiple independent lines of evidence demonstrating a reproducible abnormal biological state.
A common theme emerges from all of these definitions.
Disease is not simply the presence of an unusual molecule, an unexpected microscopic appearance or an isolated laboratory result. Rather, it represents the convergence of multiple independent lines of evidence demonstrating a reproducible abnormal biological state.
Viewed in this way, disease is not an observation. It is a classification.
This distinction is important because scientific reasoning often proceeds by gradual accumulation of evidence. An observation may suggest a possibility. Several observations may strengthen that possibility. Structural studies may support a provisional interpretation. Functional studies may establish biological significance. Clinical studies may demonstrate reproducible consequences. Only when these independent strands converge does the evidence justify classification as a disease.
This progression can be illustrated by considering the study of amyloid.
Suppose a specimen is found to contain regions of β-sheet structure. That observation immediately tells us something about protein secondary structure, but nothing more.
Further investigation may demonstrate the presence of cross-β architecture, allowing the possibility of amyloid to be considered. Additional structural evidence may eventually justify classification as amyloid. Even then, however, another logical step remains.
Demonstrating amyloid does not, in itself, establish the existence of an amyloid disease.
Each step in that progression requires additional evidence.
β-sheet does not automatically imply cross-β architecture.
Cross-β architecture does not automatically imply amyloid.
Amyloid does not automatically imply disease.
The logical distinction is no different from many familiar examples elsewhere in medicine. The demonstration of Gram-positive cocci under the microscope does not diagnose infective endocarditis. An elevated C-reactive protein concentration does not diagnose rheumatoid arthritis. Likewise, the observation of β-sheet structure does not diagnose an amyloid disease.
There is another important consideration.
Modern biology has shown that amyloid is not inherently pathological. Numerous proteins throughout nature adopt amyloid structures as part of their normal physiological function. Hormone storage, pigmentation, innate immunity, reproduction and memory all make use of highly ordered protein assemblies that would satisfy structural definitions of amyloid. Amyloid, therefore, is a structural description, not a diagnosis.
Consequently, even if a biological specimen were unequivocally demonstrated to possess classical cross-β amyloid architecture, that finding alone would still not establish disease. One would still need to demonstrate reproducible pathology, functional impairment, clinical consequences, a recognisable natural history and, ideally, an underlying biological mechanism. Only then would the available evidence justify classification as a disease.
This distinction has important implications that extend well beyond amyloid biology. Scientific literature sometimes drifts gradually from observation to conclusion through a series of individually reasonable but collectively unwarranted inferences. A structural observation becomes a structural classification. The classification becomes interpreted as pathology. Pathology becomes assumed to represent disease. Each individual step appears modest. Taken together, they may carry the reader far beyond what the original evidence actually demonstrated.
Perhaps this is an example of what might be called evidential drift—the subtle tendency for terminology to become progressively more certain than the observations upon which it was originally based.
The lesson is not that we should become reluctant to classify. Classification is one of the great achievements of science. Rather, the lesson is that classification should always remain proportionate to the available evidence. Words should describe what has been demonstrated, not what we hope may ultimately prove to be true.
In the end, the concept of disease reminds us of a broader scientific principle.
A disease is not defined by the presence of a molecular characteristic alone, but by the demonstration that a reproducible biological abnormality gives rise to a characteristic pattern of pathological change and functional disturbance. Molecular observations may contribute to that conclusion, but they do not, in isolation, establish it.
Perhaps that is the enduring lesson. Scientific terminology should remain the servant of observation, never its master. Reality comes first. Evidence follows. Classification comes later. Only then should we decide what to call it.
And that’s Signal Over Noise today.