Table of Contents
Basic Concepts of Disease and Health
In biology and medicine, words such as “health,” “disease,” “infection,” or “pathogen” have more precise meanings than in everyday language. This chapter clarifies central terms and how they relate to each other. The biological mechanisms and examples mentioned here are treated in detail in later chapters.
Health and Normal Function
In a biological sense, an organism is considered healthy when:
- Its structures (cells, tissues, organs) are intact.
- Its functions (metabolism, regulation, reproduction, repair) proceed within a normal range for the species, age, and situation.
- It can adapt to usual environmental stresses (e.g., temperature changes, moderate infections, physical exertion) without lasting damage.
Important aspects:
- Homeostasis: dynamic balance of internal conditions (e.g., body temperature, pH, blood sugar) within a narrow range.
- Adaptation reserve: the ability to react to disturbances (stress, minor injuries, mild infections) and restore balance.
“Normal” does not necessarily mean “perfect.” Many people live in good subjective health despite minor limitations (e.g., mild allergies, corrected visual defects).
Disease and Disorder
A disease (illness) is a disturbance of structure and/or function that:
- Exceeds normal fluctuations,
- Impairs well‑being, performance, or development, and
- Usually requires or would benefit from adaptation or repair.
Biology often uses the more neutral term disorder when no clear cause is known or when value judgments are to be avoided.
Typical elements of a disease description:
- Etiology (cause): why the disease arises (e.g., virus, gene mutation, poisoning, deficiency, autoimmunity).
- Pathogenesis: how the disease develops in the body from the first disturbance to full manifestation.
- Morphology: structural changes (e.g., inflamed tissue, tumors, damaged cells).
- Function: disturbed processes (e.g., reduced lung volume, impaired hormone production).
A disease can:
- Be limited to one organ (e.g., pneumonia),
- Involve several organs (e.g., metabolic syndrome),
- Affect the entire organism (e.g., sepsis).
Health–Disease Continuum
Health and disease are not strictly separate states but extremes of a continuum:
- Asymptomatic: measurable changes are present (e.g., elevated blood pressure), but no complaints yet.
- Subclinical: mild or early changes that have not yet led to clear disease symptoms.
- Clinical: changes are so pronounced that signs and symptoms are evident.
An individual can move along this continuum over time; many diseases develop gradually.
Symptoms, Signs, and Syndromes
To describe diseases, several related terms are important:
- Symptom: A subjective or objective indication of a disorder.
- Subjective symptom: only the affected person notices it (e.g., pain, nausea, dizziness).
- Objective sign (clinical sign): can be detected by others through examination, tests, or instruments (e.g., fever, rash, elevated blood sugar).
- Syndrome: A characteristic combination of several symptoms and signs that occur together more often than by chance (e.g., metabolic syndrome). A syndrome can have different causes but a similar pattern of complaints and findings.
- Course of disease:
- Acute: starts suddenly, often severe, relatively short duration (days to weeks).
- Chronic: develops slowly or lasts a long time (months to years), often with phases of improvement and deterioration.
- Subacute: between acute and chronic; not as abrupt as acute, not as prolonged as chronic.
- Recurrent: keeps returning after symptom‑free intervals (e.g., recurrent herpes infections).
Causes of Disease: Endogenous and Exogenous
Causes (etiology) can be broadly divided into:
Endogenous Factors (Internal Causes)
These originate within the organism:
- Genetic factors: inherited or newly arisen gene mutations, chromosomal changes.
- Developmental disorders: disturbances during embryonic or fetal development.
- Endocrine and metabolic factors: hormone imbalances, enzyme defects, metabolic diseases.
- Immune factors: autoimmunity, hypersensitivity, immunodeficiency not caused by infection.
- Degenerative processes: wear, aging, cell damage without external infectious cause.
Exogenous Factors (External Causes)
These act from outside the organism:
- Living agents (biological): viruses, bacteria, fungi, protists, animals (e.g., worms, mites). These are called pathogens (see below).
- Physical factors: mechanical trauma (injuries), radiation, extreme temperature, pressure.
- Chemical factors: poisons (toxins), pollutants, drugs, alcohol, tobacco, heavy metals.
- Nutritional factors: deficiency (e.g., vitamins, minerals), excess (e.g., energy‑rich diet leading to obesity).
- Psychosocial factors: stress, chronic overload, social isolation (in humans) that can influence physiological regulation.
Many diseases result from an interaction of several factors (multifactorial origin), not from a single cause.
Pathogens and Host
A pathogen is a biological agent that can trigger disease in a host organism.
- Host: the organism in which a pathogen lives, multiplies, or from which it obtains nutrients.
- Pathogenicity: basic ability of a microorganism to cause disease.
- Virulence: degree of pathogenicity; how severe the disease is that a specific strain can cause.
- Opportunistic pathogen: normally harmless or weakly harmful organism that causes disease mainly when the host’s defenses are weakened or barriers are damaged.
Later chapters will consider specific groups of pathogens (viruses, bacteria, etc.) and how they interact with the host.
Infection, Infectious Disease, and Contagiousness
Several related but not identical terms describe the interaction with pathogens:
- Infection: the invasion and multiplication of pathogens in the body.
- Does not always lead to disease; many infections remain unnoticed.
- Infectious disease: illness that arises when an infection sufficiently disturbs body functions.
- Example: infection with influenza virus → development of flu.
- Contagiousness (infectivity): how easily a pathogen is transmitted from one host to another.
- Latent infection: pathogens remain in the body in a dormant or low‑activity state; no or only mild symptoms, but reactivation is possible.
- Carrier state: person or animal harbors pathogens and can transmit them without being clearly ill.
The chapter “Infectious Diseases in Humans” examines transmission routes and typical disease progressions.
Acute, Chronic, and Latent Infections
When describing infections, duration and pattern are important:
- Acute infection:
- Rapid onset, pronounced symptoms.
- Usually ends with:
- Recovery (pathogens eliminated or controlled), or
- Death of the host in severe cases.
- Chronic infection:
- Pathogens or their effects persist for a long time.
- Symptoms can be mild, fluctuating, or slowly progressive.
- Latent infection:
- Pathogens remain in the body but are temporarily inactive or at low activity.
- Disease symptoms can reappear when reactivated.
These patterns are not limited to microbial diseases; similar course terms are used for non‑infectious diseases.
Contamination, Colonization, and Infection
It is important to distinguish between mere contact with microbes and actual disease:
- Contamination:
- Short‑term presence of microorganisms on surfaces, skin, or objects.
- No invasion into tissues, no multiplication in the host assumed.
- Example: bacteria on hands after touching contaminated surfaces.
- Colonization:
- Long‑term settlement and multiplication of microorganisms on body surfaces (e.g., skin, intestinal mucosa) without damage.
- Many colonizing microbes are part of the normal microbiota and can even be useful (e.g., intestinal bacteria producing vitamins).
- Infection:
- Microbes invade tissues, multiply there, and can trigger a reaction of the host.
So: Not every microbe we encounter leads to infection, and not every infection leads to disease.
Endemic, Epidemic, and Pandemic
To describe how widespread an infectious disease is in populations:
- Endemic:
- Occurs permanently in a region or population, often at relatively constant levels.
- Example: certain parasitic diseases in specific tropical regions.
- Epidemic:
- A sudden and clearly increased occurrence of a disease within a limited region and time period, beyond the expected background level.
- Pandemic:
- Epidemic that spreads across countries and continents and affects a large part of the world.
These terms relate to the spread in populations, not to the severity of individual cases.
Risk Factors and Predisposition
Certain characteristics or behaviors increase the likelihood of disease:
- Risk factor:
- Feature that is statistically associated with a higher probability of disease.
- Can be modifiable (e.g., smoking, dietary habits) or non‑modifiable (e.g., age, sex, specific genes).
- Predisposition:
- Increased susceptibility to a disease due to genetic constitution or early influences.
- Not a guarantee of disease, but increased risk.
From a biological perspective, risk factors and predispositions change the interaction between organism and environment, often by modifying the functioning of regulatory systems.
Defense, Resistance, and Immunity
To understand infectious diseases, several basic terms for defense are needed:
- Defense mechanisms: all processes by which the body recognizes, neutralizes, or eliminates harmful influences (physical, chemical, biological).
- Resistance:
- General ability of an organism to repel or tolerate harmful agents.
- Can be innate (e.g., skin as a barrier) or acquired (e.g., strengthened through previous exposure).
- Immunity:
- Specific, mostly long‑lasting protection against particular pathogens or their toxins.
- Often based on the specific immune system (e.g., antibodies, memory cells).
The details of non‑specific and specific immune responses, as well as vaccinations, are explained in the “Immunobiology” chapter.
Prognosis, Complication, and Outcome
Finally, some general terms about the expected course and outcome:
- Prognosis:
- Probable course and outcome of a disease under certain conditions (with or without treatment).
- Can be favorable, uncertain, or unfavorable.
- Complication:
- Unfavorable additional event in the course of a disease (e.g., secondary infection, organ failure) that is not a typical part of the normal course.
- Remission:
- Temporary or permanent reduction or disappearance of disease symptoms.
- Relapse (recurrence):
- Return of a disease after a phase of apparent recovery.
These terms help to describe and assess the temporal development of diseases, independent of their specific causes.
This terminology forms the foundation for the later chapters on infectious diseases, immunobiology, and health‑related environmental issues.