The natural history of a disease refers to its progression over time in an individual, in the absence of treatment or prevention. It consists of two phases: prepathogenesis, where exposure factors are present in the environment, and pathogenesis, where the disease process occurs in the person. The natural history describes the subclinical phase, clinical symptoms, potential complications, and prognosis or outcome of the disease. Understanding the natural history is important for developing prevention strategies, though implementing those strategies can face challenges.

1. NATURAL HISTORY OF
DISEASE
Based on K Park Preventive and Social Medicine 25e

2. Introduction
● Definition of natural history of disease
● Factors influencing natural history
● Stages of natural history
● Examples of diseases and their natural history
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3. ● Disease results from a complex interaction between man, an agent (or cause of disease)
and the environment.
● Then 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 stage of its prepathogenesis phase to
its termination as recovery, disability or death, in the absence of treatment or prevention.
● Each disease has its own unique natural history, which is not necessarily the same in all
individuals, so much so, any general formulation of the natural history of disease is
necessarily arbitrary.
● The natural history of disease is best established by cohort studies.
● What the physician sees in the hospital is just an "episode" in the natural history of disease.
The epidemiologist, by studying the natural history of disease in the community setting, is in
a unique position to fill the gaps in our knowledge about the natural history of disease.

4. The natural history of disease refers to the
progression of a disease process in an individual over
time, in the absence of treatment. It consists of two
phases: prepathogenesis (i.e., the process in the
environment) and pathogenesis (i.e., the process in
man)1. The inception of a disease is not a firmly
defined concept1. The natural history of a disease is
sometimes said to start at the moment of exposure to
or accumulation of factors capable of causing diseas

6. Pre-pathogenesis Phase
● Host factors
● Environmental factors
● Agent factors
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8. • 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 favour 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 disease".
• Potentially we are all in the prepathogenesis phase of many diseases, 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. The
mere presence of agent, host and favourab le environmental factors in the
prepathogenesis period is not sufficient to start the disease in man. What is required
is an interaction of these three factors to initiate the disease process in man.
• The agent, host and environment operating in combination determine not only the
onset of disease which may range from a single case to epidemics (as depicted in
Fig. 9's black area) but also the distribution of disease in the community.

9. Pathogenesis Phase
● Subclinical phase
● Clinical phase
● Complications
● Prognosis
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10. • 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
diseases, i.e. , the disease agent multiplies and induces tissue and
physiological changes, the disease progresses through a period of
incubation and later through early and late pathogenesis.
• The final outcome of the disease may be recovery, disability or death.
• The pathogenesis phase may be modified by intervention measures such
as immunization and chemotherapy

11. • It is useful to remember at this stage that the host's reaction to infection with a disease agent is
not predictable.
• That is, the infection may be clinical or subclinical; typical or atypical or the host may become a
carrier with or without having developed clinical disease as in the case of diphtheria and
hepatitis B.
• In chronic diseases (e.g. , coronary heart disease, hypertension , cancer), the early
pathogenesis phase is less dramatic. This phase in chronic diseases is referred to as pre-
symptomatic phase.
• During the pre-symptomatic stage, there is no manifest disease. The pathological changes are
essentially below the level of the "clinical horizon". The clinical stage begins when recognizable
signs or symptoms appear. By the time signs and symptoms appear, the disease phase is
already well advanced into the late pathogenesis phase. In many chronic diseases, the agent-
host environmental interactions are not yet well understood.

12. Agent Factors
A disease may have a single agent, a number of independent alternative agents
or a complex of two or more factors whose combined presence is essential for the
development of the disease.
• Biological Agents: Infectivity, Pathogenecity, Virulence (Case fatality rate)
• Nutrient Agents: Protein energy malnutrition (PEM), anaemia, goitre, obesity and vitamin
deficiencies are some of the current nutritional problems in many countries.
Physical Agents: Exposure to excessive heat, cold, humidity, pressure, radiation, electricity. sound,
etc may result in illness.
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13. Clinical Spectrum
● Patterns of illness
● Severity of illness
● Outcomes
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14. Diagnostic Phase
● Screening
● Definite diagnosis
● Differential diagnosis
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15. Therapeutic Phase
● Drug therapy
● Non-drug therapy
● Prophylaxis
● Rehabilitation
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16. Epidemiological Phase
● Patterns of occurrence
● Epidemic
● Pandemic
● Endemic
● Sporadic
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17. Outcome Phase
● Mortality
● Morbidity
● Recovery
● Chronicity
● Disability
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18. Conclusion
● Importance of understanding natural history
● Prevention strategies
● Challenges in implementing prevention
strategies
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