Week 2

Concepts of Health and Disease

• Webster defines disease as a condition impairing body health, deviating from a healthy state, and altering human body functions.
• The simplest definition of disease is the opposite of health—any deviation from normal physical or mental well-being.

Concepts of Causation

• Before Louis Pasteur, various theories explained disease causation, including supernatural and humoral theories.
• Microbiology discoveries marked a turning point in understanding disease causes.

Germ Theory of Disease

• The germ theory gained prominence in the late 19th and early 20th centuries, shifting focus from environmental factors to microbes as the primary disease cause.
• The germ theory often describes a one-to-one relationship between an infectious agent and disease.
• Current understanding recognizes that diseases are usually caused by multiple factors.

Epidemiological Triad

• The germ theory has limitations. Exposure to an infectious agent does not always lead to disease.
• Other factors, such as host susceptibility and environmental conditions, are crucial in disease development.
• The epidemiological triad considers the agent, host, and environment as key factors in disease causation.
• The interaction of these three factors is essential for disease onset.

Agent Factors

• Biological agents include viruses, bacteria, fungi, protozoa, and metazoa, exhibiting infectivity, pathogenicity, and virulence.
• Nutritional agents include protein, fat, carbohydrates, vitamins, minerals, and water. Imbalances cause nutritional disorders.
• Physical agents encompass extreme temperatures, humidity, pressure, radiation, electricity, and sound.
• Chemical agents include both endogenous (body-produced) and exogenous (external) substances. Examples include urea, ketones, metals, allergens, fumes, and dust.
• Mechanical forces such as friction and crushing injuries can also cause disease.
• Social agents such as smoking, poverty, lifestyle choices, and social isolation are also considered factors.

Host Factors

• Host factors are characteristics of the person/animal susceptible to infection.
• These can include demographic characteristics (age, sex) and biological characteristics (genetics).

Environmental Factors

• The physical environment includes air, water, soil, and living conditions.
• The biological environment includes other living things.
• Psychosocial environments include social, economic, and cultural conditions.

Multifactorial Causation

• Advances in medicine showed that diseases of civilization (e.g., cancer, heart disease) are often multifaceted rather than solely caused by single causes.
• Various factors including genetic, psychological, behavioral & social elements contribute to diseases.

Web of Causation

• The web of causation model describes multiple factors interacting to cause diseases, like chronic illnesses.
• The web emphasizes the interconnectedness of predisposing factors in such diseases, not a single cause.
• Elimination or control of critical links in the web is often sufficient to control the illness.

Natural History of Disease

• Disease development involves a complex interplay among host, agent, and environment.
• The natural history of disease describes how a disease progresses over time from pre-pathogenesis to outcomes like recovery, disability, or death.

Prepathogenesis/ Pathogenesis

• The pre-pathogenesis phase is the period before disease onset. Factors present in the environment support infectious agent interaction with susceptible hosts
• In the pathogenesis phase, the infectious agent enters the host, leading potentially to disease with progression through incubation to early, late stages, possible outcomes like recovery, disability, or death. Disease processes may be modified by intervention.

Risk Factors

• Risk factors are attributes or exposures associated with disease development.
• Their presence doesn't guarantee disease, but their absence doesn't rule it out.
• Identifying risk factors improves disease prevention and intervention strategies.

Spectrum of Disease

• The spectrum of disease model shows disease manifestations with severity on a scale, mild to severe.
• Subclinical infections exist on one end, and severe, potentially fatal illnesses are on the other.
• States of immunity and receptivity affect the expression of disease manifestations

Iceberg of Disease

• The iceberg model presents hidden disease cases (i.e., those undiagnosed), alongside observable overt cases.
• It highlights the large portion of disease presence that isn't readily apparent in a population.
• Detection and control of hidden diseases are important for effective preventive medicine strategies.

Disease Control

• Disease control aims to reduce disease incidence, duration, transmission, complications, and financial burden.
• Control strategies may focus on preventing disease (primary prevention) or reducing its impact after it occurs (secondary prevention).

Disease Elimination

• Elimination of disease interruption focuses on stopping disease transmission within defined regional areas.

Disease Eradication

• Eradication aims to eliminate a disease completely from the entire global population by eradicating the infectious agent.
• Smallpox is the only disease eradicated till date.

Screening for Diseases

• Screening is a method for detecting unrecognized diseases in individuals with no symptoms.
• Screening tests are designed to detect conditions in the early stages, while the disease is still reversible.
• Criteria for screening include the disease's importance, recognizable latent stage, and an understanding of its natural history.

Test Acceptability, Repeatability, and Validity

• Screening tests should be acceptable to the screened population, repeatable in their results, and valid in measuring the target condition.

Evaluation of Screening Tests

• Sensitivity, specificity, predictive values of positive and negative tests are used to evaluate screening tests
• These measure the tests' accuracy in identifying diseased and non-diseased individuals.

Uses of Screening

• Screening detects unrecognised diseases, controls disease spread, helps researchers learn more about a disease’s natural history, and educates the public.

Infection - Definition, Dynamics, and Sources

• Infection is the entry and establishment of an infectious agent in the body.
• Disease transmission involves a reservoir of infection, transmission methods, and a susceptible host.
• Reservoirs of infection can be humans, animals, or nonliving objects like soil.

Modes of Transmission

• Infectious diseases can be transmitted directly (contact, droplets) or indirectly (vehicles, vectors, air, fomites).

Susceptible Host

• Infection/disease stages are: Incubation, onset/prodromal, advance/fastigium, defervescence, and convalescence.
• Stages from infection to overt disease.

Active Immunity

• Active immunity develops through infection or vaccination, leading to the production of antibodies and immune cells.

Passive Immunity

• Passive immunity provides temporary protection through pre-formed antibody transfer.
• This type of immunity, from maternal transfer or pre-made antisera, is temporary.

Explore the fundamental concepts of health, disease, and causation through various historical perspectives. Understand the transition from early theories to germ theory and its implications in modern epidemiology. This quiz covers key definitions and frameworks essential for studying health sciences.