Table of Contents
Biology does not deal with “health” and “disease” as purely personal or moral categories, but as biological states of living organisms. This section provides the framework needed to understand the later, more specific chapters on infectious diseases, immunobiology, and plant defense.
Biological Perspectives on Health and Disease
From a biological point of view, every organism is constantly exposed to internal and external influences that can disturb its normal functioning. “Health” and “disease” describe different regions of this continuum:
- Health refers to a state in which the organism’s structures and functions are sufficiently intact that it can survive, reproduce, and respond appropriately to environmental challenges.
- Disease refers to a disturbance of structure and/or function that exceeds the organism’s capacity to compensate, leading to reduced performance, suffering, or increased risk of death.
In reality, many states lie between these poles: an organism can appear healthy but carry pathogens, or function relatively well despite chronic impairment. Thus, rather than two sharply separated conditions, health and disease form a spectrum.
Levels of Disturbance: From Molecules to the Whole Organism
Because organisms are hierarchically organized, disturbances can begin at different levels and then spread:
- Molecular level: Changes to DNA, proteins, lipids, or other molecules (e.g., mutations, misfolded proteins, toxins) can impair cellular processes.
- Cellular level: Cells may die too quickly (degeneration), divide uncontrollably (cancer), or fail to perform their special tasks (e.g., insulin-producing cells in diabetes).
- Tissue and organ level: Damage accumulates at the level of tissues (e.g., muscle, epithelium, nerve tissue) and organs (e.g., liver, heart, brain), altering their function.
- Organ system level: Entire systems, such as the circulatory or nervous system, may be affected, with consequences for the whole organism.
- Whole-organism level: Disturbances manifest as recognizable signs and limitations in behavior, performance, and survival.
Health, in this framework, corresponds to the relative integrity and coordination of all these levels.
Internal and External Causes
Biology distinguishes broadly between causes that arise from within the organism and those coming from the outside. These categories often interact.
Internal (Endogenous) Factors
Endogenous causes develop from the organism’s own genetic makeup and internal processes, for example:
- Genetic predisposition: Variations (alleles) that increase susceptibility to certain diseases.
- Spontaneous changes: Mutations or errors in cell division that can lead to disorders such as cancer.
- Regulatory disturbances: Imbalances in endocrine, nervous, or immune regulation (e.g., autoimmune reactions, hormone overproduction).
Such factors do not always lead to disease; the environment and lifestyle often determine whether a predisposition is expressed.
External (Exogenous) Factors
Exogenous causes originate in the environment. They include:
- Biological agents: Viruses, bacteria, fungi, protists, and animals that act as parasites or pathogens.
- Physical factors: Extreme temperatures, radiation, mechanical trauma.
- Chemical factors: Poisons, pollutants, drugs (therapeutic or abused substances), allergens.
- Nutritional and lifestyle factors: Quality and quantity of food, physical activity, sleep, exposure to stressors.
Many diseases arise when exogenous factors act on an organism with specific endogenous vulnerabilities.
Concepts of Normality and Homeostasis
The assessment of health and disease is closely tied to the concept of homeostasis: the ability of an organism to maintain stable internal conditions despite changing external conditions.
Examples of homeostatically regulated variables include:
- Body temperature
- Blood pH
- Osmotic pressure and ion concentrations
- Blood glucose concentration
Health is often associated with the capacity to keep these variables within limits appropriate for the species. Disease arises when regulation fails or must operate at the edge of its capacity. Short-term deviations (e.g., elevated temperature during fever) can also be part of an active defense reaction and are not necessarily “unhealthy” in the evolutionary sense.
Symptoms, Signs, and Course of Disease
To describe and study disease scientifically, biology and medicine distinguish several aspects:
- Symptoms: Subjective perceptions of the organism (e.g., pain, tiredness, nausea). In human medicine, this refers to what patients report.
- Signs: Objectively observable changes (e.g., rash, swelling) or measurable changes in body functions (e.g., blood pressure, laboratory values).
- Syndromes: Typical combinations of signs and symptoms that tend to occur together.
The course of a disease can be characterized by:
- Acute: Sudden onset, short duration.
- Chronic: Long-lasting, often with slow onset.
- Recurrent: Repeated episodes separated by symptom-free intervals.
- Latent or subclinical: Processes present but without clear symptoms.
The organism’s responses (e.g., fever, inflammation, behavioral changes such as withdrawal and rest) can be viewed as adaptive strategies shaped by evolution to improve survival.
Interaction of Biological, Environmental, and Social Factors
Even though this course focuses on biological aspects, real health and disease states are strongly influenced by environmental and social conditions:
- Living conditions: Nutrition, housing, hygiene, access to clean water, exposure to pollutants.
- Social factors: Stress, social support, health behavior patterns spread via culture.
- Healthcare systems: Prevention (e.g., vaccination), early detection, and treatment.
From a population perspective, the frequency and distribution of diseases (epidemiology) are partly determined by such conditions. Biological knowledge is therefore essential not only for understanding individual disease mechanisms, but also for planning preventive and public health measures.
Health, Disease, and Evolution
From an evolutionary viewpoint, several aspects are important:
- Trade-offs: Traits that are advantageous in one context can increase disease risk in another (e.g., strong immune responses help fight infections but may predispose to autoimmunity or allergies).
- Mismatch: Biological adaptations are tuned to past environments. Rapid environmental changes (dietary changes, altered activity levels, artificial light) can create mismatches that contribute to “modern” diseases.
- Pathogens as evolving organisms: Pathogens themselves evolve (e.g., resistance to antibiotics or antivirals), influencing the dynamics of infectious diseases.
Health can thus be seen as a dynamic balance achieved under specific environmental conditions and shaped by evolutionary history.
Overview of the Following Chapters
The subsequent chapters in this section build on this general framework:
- Terms and Definitions: Refines and formalizes key concepts introduced here.
- Infectious Diseases in Humans: Examines diseases caused by subviral agents, viruses, bacteria, fungi, protists, and animals, including transmission and typical courses.
- Immunobiology: Deals with the body’s defense mechanisms against pathogens and how specific and nonspecific immune responses operate.
- Plant Defense Systems: Extends the concept of disease and defense to plants and explains how they respond to attackers and environmental stress.
Together, these chapters integrate the concept of disease and health from the molecular to the ecological and societal levels.