concept of disease
DESCRIPTION
CONCEPT OF DISEASETRANSCRIPT
DISEASE• Disease is a
physiological / psychological dysfunction.
• Illness is a subjective state of a person who feels aware of not being well.
• Sickness is a state of social dysfunction, i.e., a role that the individual assumes when ill (“sickness role”).
CONCEPT OF CAUSATION
• Germ Theory of Disease.• Epidemiological triad.• Multi factoral
causation.• Web of causation.
GERM THEORY OF DISEASE
• This concept gained momentum during the 19th
• The disease model accordingly is as follows.
WEB OF CAUSATIONDisease agent
Man Disease
The concept of cause embodied in the germ
theory of disease is referred to as a one-to-
one relationship between causal agent and
disease
WEB OF CAUSATION
EPIDEMIOLOGICAL TRIAD• The germ theory has many limitations.
• It is well known that not everyone exposed to tuberculosis develops tuberculosis, however in an undernourished or otherwise susceptible person may result in clinical disease.
• There are other factors relating to the host & environment which are equally important to determine whether or not disease will occur in the exposed host.
• This demanded a broader concept of disease causation that synthesized the basic factors of agent, host, & environment
AGENT HOST
ENVIRONMENT
EPIDEMIOLOGICAL TRIAD
• The Agent, Host, Environment model has been in use for many years.
• It has helped epidemiologist to focus on different classes of factors especially with regard to infectious diseases
MULTIFATORIAL CAUSATION• Pettekofer of Munich (1819 – 1901)
propounded the concept of multi factorial causation.
• Modern diseases such as coronary heart diseases, chronic bronchitis, mental illness could not be explained by the germ theory of disease.
MULTI FACTORIAL CAUSATION
• This realization began to draw that a single cause idea was an over simplification & that the other factors in the etiology of disease – social, economic, cultural, genetic & psychological which are equally important.
• E.g., Tuberculosis is not merely due to tubercle bacilli; factors such as poverty, overcrowding & malnutrition contribute to its occurrence.
• The purpose of knowing the multiple factors of disease is to quantify and arrange them in priority sequence for modification or amelioration to prevent or control disease.
• The multi factorial causation offers multiple approaches for the prevention \ control of disease
WEB OF CAUSATION• Mac Mahon & Pugh suggested this
theory.
• This model is ideally suited for the study of chronic diseases where disease agent is often not known, but is the outcome of interaction of multiple factors.
• The web of causation considers all the predisposing factors of any type & their complex interrelationship with each other.
• The causal web provides a model which shows a variety of possible interventions that could be taken which might reduce the occurrence of the disease.
• Some times removal or elimination of just only one link or chain may be sufficient to control disease, provided that link is sufficiently important in the pathogenic process.
• The relative importance of theses factors may be expresses in terms of “relative risk”.
NATURAL HISTORY OF DISEASE
• Disease results from a complex interaction between man, an agent & the environment.
• “NATURAL HISTORY OF DISEASE” is a key concept in epidemiology.
• It signifies the way in which the disease evolves over time from the earliest stage of its pre pathogenesis phase to its termination or prevention.
• Each disease has its own unique natural history.
• It is customary to describe the natural history of a disease as consisting of two phases;
1. PRE PATHOGENESIS PHASE. 2. PATHOGENESIS PHASE.
THE PRE PATHOGENESIS PHASE• This refers to the preliminary period
to the onset of the disease in man.
• The disease agent has not yet entered man, but the factors which favour interaction with human host are already existing in the environment
• This situation is referred to as “man in the midst of disease”.
• The causative factors of disease may be classified as agent, host & environment.
• These factors are referred to as EPIDEMIOLOGICAL TRIAD”.
• The mere presence of these factors is not sufficient to start the disease.
• What is required is the interaction of these three factors.
PATHOGENESIS PHASE• The pathogenesis phase begins with
the entry of the disease agent in the susceptible human host.
• The further events in the pathogenesis phase are clear cut in infectious disease.
• The agent multiplies & induces tissue & physiological changes, the disease progresses through a period of incubation & later through early & late pathogenesis.
• The final outcome of the disease may be modified by intervention measures such as immunization & chemotherapy.
COURSE OF DISEASE
AGENT FACTORS BIOLOGICAL AGENTS
BIOLOGICAL AGENTS
• Virus, bacteria, rickettsiae, fungi,, protozoa & metazoa.
• These agents exhibit certain host related biological properties such as
CHARACTERISTICS OF AGENT FACTORS
INFECTIVITY : Ability of the infectious agent to invade &
multiply in a host.
PATHOGENECITY : Ability to induce clinically apparent
illness .
• VIRULENCE : The proportion of clinical cases
resulting in severe clinical
manifestations.
NUTRIENT AGENT• These are proteins, fats,
carbohydrates, vitamins, minerals & water.
• Any excess or deficiency of the intake of nutritive elements may result in nutritional disorders,
• PEM, anaemia, goitre, obesity & vitamin deficiencies are some of the current nutritional problems in our country.
PHYSICAL AGENT
• Exposure to excessive heat, cold, humidity, pressure, radiation, electricity, sound, etc may result in illness
CHEMICAL AGENTS• ENDOGENOUS : Some of the
chemicals may be produced in the body as a result of derangement of function, e.g., urea (ureamia), serum bilirubin (jaundice), ketones (ketosis), uric acid (gout), calcium carbonate (kidney stones), etc.
• EXOGENOUS : Agents arising outside of human host, e.g., allergens, metals, fumes, dust, gases, insecticides, etc. These may be acquired by inhalation, ingestion or inoculation
MECHANICAL AGENTS
• Exposure to chronic friction & other mechanical forces may result in crushing, tearing, sprains, dislocations & even death.
ABSENCE OR INSUFFICIENCY OR
EXCESS OF A FACTOR NECESSARY TO HEALTH
• These may be Chemical factors : hormones (insulin,oestrogens), Nutrient factors : iron deficiency, Lack of structure : Thymus, Lack of part of structure: Cardiac defects, Chromosomal factors : Mongolism,Dawn Syndrome,Immunological factors : Agammaglobulinaemia
SOCIAL AGENTS• The social agents are
poverty, smoking, alcohol, unhealthy life styles, social isolation, maternal deprivation, etc.
HOST FACTORS• The host factors may be classified as : • 1. Demographic characteristics
such as age, sex, ethnicity.• 2. Biological characteristics such as
genetic factors, bio chemical levels of the blood (cholesterol).
• 3. Social & Economic characteristics such as education, occupation, stress, marital status, housing.
• 4. Life style factors such as personality traits, living habits, physical exercise, use of alcohol, drugs, smoking.
ENVIRONMENTAL FACTORS• The study of disease is really the
study of man & his environment.
• The environment can be classified as Internal & external environment. The external or the macro environment is defined as all that which is external to the individual human host. They are categorized as follows.
•1. Physical environment.•2.Biological environment.•3.psychosocial environment.
PHYSICAL ENVIRONMENT• The term “physical environment” is
applied to non living things & physical factors. (air, water, soil, housing, climate, geography, heat, light, noise, debris, radiation, etc.)with which man is in constant interaction.
• Man’s victory over his physical environment has been responsible for most of the improvement in health during the past century.
• However this has added to a host of other problems like air pollution, noise pollution, urbanization, radiation hazards etc, human exposure to electromagnetic energy, etc.
BIOLOGICAL ENVIRONMENT
• The biological environment is the universe of living things which surrounds man.
• The living things are bacteria, virus, insects, rodents, animals & plants.
• These are constantly working for their survival, in this process some of them act as disease producing agents, reservoirs of infection, intermediate hosts & vectors of disease.
• When the harmonious relationship is disturbed, ill health results.
PSYCHOSOCIAL ENVIRONMENT• The psychosocial environment
encompasses cultural values, customs, habits, beliefs, attitudes, morals, religion, education, lifestyles, community life, health services, social& political organization.
• Man is in constant interaction with that part of the social environment known as “people”.
• He is a member of a social group.• The behaviour of a person can affect
another.
• Conflict & tension between members of a group can yield a great distress. The impact of social environment has both positive & negative aspects on the health of individuals.
• A favorable psycho social environment has both positive & negative aspects on health of individuals & communities.
Psychosocial factors can also affect negatively man’s health & well being. For example poverty, urbanization, migration & exposure to stressful situations such as bereavement, desertion, loss of employment, birth of a handicapped child may produce feelings of anxiety, depression, anger, frustration & so forth.
• Man today is viewed as an “agent” of his own disease. His state of health is determined more by what he does to himself than what some outside germ or infectious agent does to him.
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