Concepts of disease causation DrSantosh K Yatnatti Assistant Professor Department of Community Medicine
Introduction Disease: • “A condition in which body health is impaired, a departure from the state of health, an alteration of the human body interrupting the performance of vital functions”- Webster • “ A condition of the body or some part or organ of the body in which its functions are disrupted or deranged”- Oxford English dictionary
Ecological definition “ maladjustment of human organism to the environment” • Sociological definition “ disease is a social phenomenon, occurring in all societies and defined and fought in terms of particular cultural forces prevalent in the society”
Disease v/s illness v/s sickness: • Disease: a physiological or psychological dysfunction • Illness: a subjective state of a person who feels aware of not being well • Sickness: a state of social dysfunction i.e a role that the individual assumes when ill
A cause of a diseaseoccurrence“is an event, condition, or characteristic that preceded the disease onsetand that, had the event, condition, or characteristic been different in a specified way, the disease either would not have occurred at all or would not have occurred until some later time”. Rothman
Concepts of disease causation • Supernatural theory of disease • Tridosha theory of disease • Theory of humours • Theory of contagion/ miasma • Theory of spontaneous generation • Germ theory of disease • Epidemiological triad • Multi factorial causation, Web of causation • BEINGS model of disease causation • Epidemiological wheel • Necessary and sufficient causes • Risk factors
Supernatural theory of disease: e.g. • Possession by evil spirits, • Wrath of gods, • Punishment - evil deeds during previous births and so on • Leprosy, plague, congenital malformations etc
2. Tridosha theory of diseases: According to AURVEDA 3 doshas or humors are • vata (wind), • pitta(gall), • kapha(mucus).
4. Theory of contagion: • Contact with diseased person • Miasma theory- contact with bad clouds- ex: cholera epidemic in London • Theory of spontaneous/ de-novo generation
5. Germ theory of disease causation: - Related every human disease to a specific microbe or “germ”, • Each and every human disease -caused by a microbe or germ, which is specific for that disease.
Koch’s postulates 1. The bacterium should be constantly associated with the lesions of disease. 2. It should be possible to isolate the bacterium in pure culture from the lesions. 3. Inoculation of such pure culture into suitable laboratory animals should reproduce the lesions of the disease. 4. It should be possible to re-isolate the bacterium in pure culture from the lesions produced in the experimental animals
Drawbacks: • Non communicable diseases • Only infection, ex: TB infection
6. The Epidemiological Triad” • Role of other factors in accentuating or attenuating the effect of the “germ” or “agent” of disease. • “Epidemiological triad”- interaction between agent, host and environment
Agent: A substance- living or non living or a force tangible or intangible, the excessive presence and the relative lack of which will initiate or perpetuate a disease process Types:
Multi factorial causation of disease: Factor A+ Factor B+ Factor C Disease Ex: Infection with TB bacilli Hypertension and IHD
8. Theory of web of causation: • No single agent in the development of NCD s • Events like road traffic accidents- could not be explained
Changes in lifestyle stress • Abundance of food smoking emotional disturbance ageing • Obesity HTN • Hyperlipidemia ^thrombotic tendency changes in A walls • Coronary atherosclerisis • coronary occlusion • myocardial ischemia & MI
Risk taking behaviour Late night/ early morning Influence of alcohol Not following traffic rule Excessive speed difficulty in judging Road traffic accident
Importance: • Few “weakest links” in the inter-lacing webs, in prevention • The most important links are determined by relative risk.
9. The “BEINGS” Model of Disease Causation • Biological factors innate in a human being, Behavioural factors concerned with individual lifestyles, • Environmental factors as physical, chemical and biological aspects of environment, • Immunological factors, • Nutritional factors, • Genetic factors, • Social factors, Spiritual factors and Services factors, related to the various aspects of health care services
10. Epidemiological wheel theory: • To highlight the comparative role of “genetic” and the “environmental” in causation of disease.