Table of Contents
What Is Meant by “Disease”?
In this context, disease refers to a state in which the normal structure or function of an organism (or one of its parts) is disturbed in such a way that:
- performance and well-being are measurably impaired, and
- the disturbance cannot be explained simply as a normal variant (e.g., natural aging, minor fluctuations).
Unlike the broader concept of health, which includes subjective well-being and social aspects, disease is typically defined by recognizable changes that can, in principle, be described, measured, and classified.
Key aspects:
- Deviation from the normal state: Something in structure (morphology) or function (physiology, biochemistry) is altered.
- Reduced adaptability: The organism can no longer adequately respond to internal or external demands.
- Potential for detection: There are signs (symptoms, findings) that can be observed by the affected person or by others.
Components of Disease: Symptoms, Signs, and Syndromes
To describe a disease scientifically, several terms are useful:
- Symptom:
A subjective complaint or experience of the affected individual.
Examples: pain, fatigue, nausea, dizziness. - Sign (clinical finding):
An objective change that can be observed or measured by others.
Examples: fever (measured temperature increase), rash, elevated blood pressure, abnormal lab values. - Syndrome:
A characteristic combination of several symptoms and signs that occur together so often that they are given a name, even if the underlying cause may be diverse or not yet fully understood.
Example: Down syndrome (typical physical features and developmental characteristics associated with a defined chromosomal change).
When symptoms, signs, and possibly test results follow a typical pattern over time, we speak of a disease entity (e.g., “influenza,” “type 1 diabetes”).
Course of Disease: Acute, Chronic, Latent
Diseases can be characterized by how they develop and persist:
- Acute disease:
- Rapid onset (hours to days)
- Often severe symptoms
- Typically short duration
Example: an acute middle ear infection. - Subacute disease:
- Onset and course between acute and chronic
- Symptoms may be less dramatic but still relatively rapid in development.
- Chronic disease:
- Slow or insidious onset
- Long-lasting (months to years, sometimes lifelong)
- Often associated with permanent changes or damage
Examples: chronic bronchitis, diabetes mellitus. - Recurrent (relapsing) disease:
- Episodes (relapses) of disease separated by periods with few or no symptoms
Example: recurrent depression, relapsing malaria. - Latent disease / latent phase:
- A disease process is present but causes no noticeable symptoms for a time.
- The person may feel healthy, even though measurable changes exist or the organism is already infectious in the case of some infections.
Causes and Influencing Factors (Etiology)
A cause in biology and medicine is a factor without which a specific disease would not have occurred in that form and at that time. For most diseases, the “cause” is not a single element but a combination of:
- External (exogenous) factors:
- Physical: mechanical injury, radiation, extreme temperatures
- Chemical: toxins, drugs, pollutants
- Biological: pathogens such as viruses, bacteria, fungi, protists, parasites
- Internal (endogenous) factors:
- Genetic disposition (hereditary variants, mutations)
- Metabolic and hormonal regulation
- Immune system characteristics
- Age, sex, nutritional and training status
Often, internal susceptibility and external influences interact: the same external factor (e.g., smoking) does not lead to the same disease in every person; the likelihood and the type of disease depend on individual internal factors.
In addition to direct causes, there are risk factors that increase the probability of disease but do not inevitably cause it (e.g., high blood pressure as a risk factor for stroke).
Manifestation and Progression: From Exposure to Outcome
Typical conceptual stages in the life history of many diseases include:
- Exposure:
Contact with a potentially harmful influence (e.g., pathogen, toxin, chronic stress). - Incubation or latency period:
Time between exposure and the appearance of first symptoms. - For infections: incubation period
- For many non-infectious diseases: latency (often long, e.g., decades in some cancers).
- Prodromal stage:
Unspecific early symptoms (malaise, slight fatigue, mild fever), which precede more specific disease signs. - Manifest disease:
Full picture of the disease with characteristic symptoms and signs (e.g., typical rash, characteristic neurological failures, typical laboratory constellation). - Resolution, chronicity, or death:
- Complete recovery (restitutio ad integrum)
- Partial recovery with residual defects (restitutio cum defectu), i.e., remaining damage
- Transition into a chronic state
- Death of the organism in severe cases.
Not every disease goes through all stages in a clear form; the model mainly helps to analyze and compare patterns of progression.
Levels of Disease: From Molecules to the Whole Organism
Disease can be described on different biological levels:
- Molecular level:
Changes in DNA, proteins, signaling molecules (e.g., enzyme defect, receptor mutation). - Cellular level:
Disturbed cell functions (e.g., disrupted energy metabolism, uncontrolled cell division). - Tissue and organ level:
Structural damage (e.g., scarred heart muscle), inflammatory changes, or functional deficits (e.g., reduced pumping capacity). - Organism level:
Observable symptoms and impairments of the whole organism (e.g., reduced performance, disturbed coordination, weight loss). - Population level:
When many individuals in a population are affected, we speak of morbidity (frequency of disease) and in severe outcomes of mortality (frequency of deaths due to a disease).
Understanding that a disease can originate or manifest at one level, but have consequences at others, is central to modern biology and medicine.
Classifying Diseases
To describe and compare diseases, they can be grouped according to different criteria (without going into the specific examples that appear in other chapters):
- By cause (etiology):
- Infectious diseases (caused by pathogens)
- Genetic diseases (based on inheritable changes)
- Degenerative diseases (wear and tear, aging processes)
- Autoimmune diseases (immune system directed against the body’s own structures)
- Nutritional and metabolic diseases
- Tumor diseases (neoplasias)
- Traumatic diseases (injury-related)
- By affected system or organ:
- Cardiovascular diseases, respiratory diseases, nervous system diseases, etc.
- By mechanism of disturbance:
- Inflammatory, toxic, neoplastic, functional, etc.
These classification systems make it possible to collect data on disease frequency, research causes and risk factors, and plan prevention and treatment strategies.
Disease, Disorder, Damage: Conceptual Distinctions
Several partially overlapping terms are used to describe states of ill health:
- Disease:
A relatively well-defined pattern of disturbance with more or less typical course and manifestations. - Disorder (functional disorder):
Disturbed function that may or may not be associated with obvious structural changes. - Damage (lesion):
A structural change at the organ, tissue, or cellular level (e.g., wound, infarct scar).
In practice, these terms blur into one another, but the distinctions are helpful:
- Damage can cause a functional disorder.
- A defined pattern of damage and disorder, with typical signs and course, is then called a disease.
Disease in a Biological and Population Context
From a biological perspective, disease is not only important at the level of the individual:
- Evolutionary perspective:
Diseases can: - influence survival and reproductive success (fitness),
- shape gene frequencies in populations (e.g., via selection for resistance traits),
- act as a regulatory factor in ecosystems by influencing population densities.
- Ecological perspective:
Pathogens and hosts form dynamic systems. Epidemics in animal or plant populations can alter food webs and ecosystem structure.
Thus, “disease” is not just a medical term but a biological phenomenon that affects individuals, populations, and ecosystems.
Border Areas: Health–Disease Continuum
In reality, the boundary between “healthy” and “diseased” is often not sharp:
- Mild or early changes may be detectable by measurements but cause no subjective complaints.
- Age-typical changes can resemble those in disease, but may be classified differently based on extent and impact.
- Social and cultural factors influence where the line is drawn (e.g., classification of certain behaviors or performance variations).
Biologically, one can think of a continuum: from an optimal functional state, through decreasing adaptability and early disturbances, to fully manifest disease and possible loss of function.