What is a disease?

What is a Disease?

A disease is an abnormality of structure or function with implications for the health of an individual, traditionally defined by the presence of signs (malfunction) and symptoms (suffering), though modern frameworks increasingly recognize that disease can exist in asymptomatic individuals when biological markers indicate pathophysiological processes are present. 1

Traditional Definition Framework

The World Health Organization's International Statistical Classification of Diseases (ICD) traditionally defines disease based primarily on clinical information, requiring both:

• Signs - Observable malfunction of bodily systems 1
• Symptoms - Subjective suffering experienced by the individual 1

This clinical framework has guided disease classification for decades but does not always incorporate advances in understanding molecular mechanisms, genetic factors, and environmental interactions that cause disease and pathology. 1

Evolving Biological Definition

The National Research Council of the National Academy of Sciences proposed a new taxonomy recommending that diseases be defined not only by traditional signs and symptoms but also by their underlying biology. 1 This represents a fundamental shift in how we conceptualize disease.

Key principle: Disease can exist before symptoms appear when biological markers indicate pathophysiological processes are active. 1

The analogy to coronary heart disease illustrates this concept - individuals with coronary atherosclerosis have disease even if they never experience a myocardial infarction, and this asymptomatic disease warrants medical treatment. 1

Specific Disease Definitions from Guidelines

Chronic Kidney Disease (CKD) provides a concrete example: disease is defined as abnormalities of kidney structure or function present for ≥3 months, with implications for the health of an individual. 1 This definition explicitly recognizes that:

• Disease can occur abruptly and may resolve or become chronic 1
• Recognition of disease has implications for individuals and their care 1
• The concept evolved after recognizing how disordered structure and function affect health across a wide severity range 1

Components of Disease Definition

A comprehensive disease definition includes:

• Biological dysfunction - Impairment of functions relevant for individual survival 2
• Harmful consequences - Either subjective suffering (illness experience) or impaired activities of daily living and social participation (sickness aspect) 2
• Structural or functional abnormalities - Detectable deviations from normal that can be objectified and classified 2, 3

Critical distinction: States of human suffering that can be objectified but lack disease symptoms accompanied by illness experience or functional impairment should be termed "disorders" rather than diseases in the medical sense. 2

Practical Clinical Application

When determining if a condition constitutes a disease:

1. Identify biological abnormality - Is there demonstrable structural or functional deviation from normal? 1, 3
2. Assess health implications - Does this abnormality have consequences for the individual's health? 1
3. Evaluate harm - Is there either subjective suffering OR objective impairment of function? 2
4. Consider biomarkers - In asymptomatic individuals, do validated biological markers indicate active pathophysiological processes? 1

Common Pitfalls

Avoid medicalizing non-specific symptom aggregations - Symptoms of uncertain cause should not be given diagnostic labels that falsely imply specific disease entities, as this creates confused concepts that impede further investigation. 4 The pragmatic diagnoses of "symptom of unknown cause" or "non-disease" are preferable to falsely labeling patients with obscure or non-existent diseases. 4

Distinguish disease from disorder - Beyond medically relevant disease symptoms, clinically relevant dysfunction must also be harmful to the individual for a mental malady to be diagnosed as disease rather than merely a disorder. 2

Recognize disease as a dynamic process - Disease represents the organism's response to maintain physical, psychological, and spiritual equilibrium, not simply a state to be suppressed. 5 Understanding disease as a meaningful process of transformation can inform more effective therapeutic approaches. 5