Natural History of Disease
The term Natural history of disease is a key concept in epidemiology. It signifies the way in which a disease evolves over time from the earliest stages of its pre-pathogenesis phage to its termination as recovery, disability or death, in the absence of treatment or prevention.
The Natural history of disease is best established by cohort studies. As these studies are costly and laborious, our understanding of the natural history of disease is largely based on other epidemiological studies, such as cross-sectional and retrospective studies, undertaken in different population setting, both national and international. It is customary to describe the natural history of disease as consisting of two phases:-
1. Pre-pathogenesis (i.e., the process in the environment).
2. Pathogenesis (i.e., the process in man).
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| Types of NHD |
1. Pre-pathogenesis phase:
This refers to the period preliminary to the onset of disease in man. The disease agent has not yet entered man, but the factors which favor its interaction with the human host are already existing in the environment. This situation is frequently referred to as “man in the midst of disease” or “man exposed to the risk of the disease”. Potentially we are all in the pre-pathogenesis phase of many disease, both communicable and non-communicable.
The causative factors of disease may be classified as AGENT, HOST and ENVIRONMENT. These three factors are referred to as Epidemiological Triad.
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| Epidemiological Triad |
2. Pathogenesis phase:
The pathogenesis phase begins with the entry of the disease “agent in the suspectable human host”. The further events in pathogenesis phase are clear cut in infectious disease, the disease progresses through a period of incubation later through early and late pathogenesis. The final outcome of disease may be recovery, disability or death. The pathogenesis phase may be modified by intervention measures such as immunization and chemotherapy.
In chronic disease (e.g. Coronary heart disease, hypertension, cancer), the early pathogenesis phase is less dramatic. This phase is chronic disease is referred to as presymptomatic phase. During presymptomatic stage, there is no manifest disease. The pathological changes are essential below the level of “clinical horizon”.
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| Natural History of Disease |
Agent factors:
The first link in the chain of disease transmission is a disease agent. Disease agent may be classified broadly into the following groups-
1. Biological agents: These are living agent of disease, virus, fungi, bacteria, protozoa and metazoa. This agents exhibit certain “host-related” biological properties such as-
i) Infectivity- This is the ability of an infectious agent to invade and multiply in a host.
ii) Pathogenicity- This is the ability induce clinically apparent illness.
iii) Virulence- This is defined as the proportion of clinical cases resulting in severe clinical manifestation.
2. Nutrient agents: These can be protein, fats, carbohydrate, vitamins, minerals and water.
3. Physical agents: Exposure to excessive heat, cold, humidity, pressure, radiation, electricity, sound etc. may result in illness.
4. Chemical agents:
i) Endogenous- Some of the chemicals may be produced in the body as a result of derangement of function, e.g., urea, uric acid, serum, bilirubin, ketones, calcium carbonate.
ii) Exogenous- Agent arising outside of human host e.g., allergens, metals, fumes, dust, gases, insecticide, etc.
5. Mechanical agents: Exposure to chronic friction and other mechanical forces may result in crushing, tearing, sprains, dislocation and even death.
6. Absence or insufficiency or excess of a factor necessary to health: These may be-
i) Chemical factors
ii) Nutrient factors
iii) Lack of structure
iv) Lack of part of structure
v) Chromosomal factors
vi) Immunological factors
7. Social agents: It also necessary to consider social agents of disease. These are poverty, smoking, abuse of drug and alcohol, unhealthy lifestyle etc.
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| Agent factors |
Host factors (intrinsic):
In epidemiological terminology, the human host is referred to as “soil” and the disease agent as “seed”.
The host factors may be classified as-
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| Host factors |
Environmental factors (extrinsic):
The study of disease is really the study of man and his environment.
For descriptive purposes, the environment of man has been divided into three components-
i) Physical environment
ii) Biological environment
iii) Psychosocial environment
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| Environment factors |