1. Epidemiologic Triads & Natural History of Disease
Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood
KSU Department of Family & Community Medicine
September, 2013

2. OBJECTIVES OF THE LECTURE
By the end of this lecture students will be able to:
Explain epidemiologic triads as a model of study of disease causation
Describe natural history and spectrum of infectious diseases and their implications for public health.
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3. Triads & Natural History
Concepts
Descriptive epidemiological triad
Analytical epidemiological triad
Natural history of disease
Spectrum of disease
Public health implications
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4. Purpose of studying causal models
Studying how different factors can lead to ill health generates knowledge for disease prevention & control
The classic epidemiological triangle or triad help understand the relation between a disease, disease causing agent and environment 4
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5. Epidemiological Triads
Descriptive Epidemiology Triad:
Person
Place
Time
Analytical Epidemiology Triad:
Agent
Host
Environment
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6. Descriptive Epidemiology
Descriptive Epidemiology is a Necessary Antecedent Of Analytic Epidemiology
To undertake an analytic epidemiologic study you must first:
Know where to look
Know what to control for
Be able to formulate hypotheses, compatible with laboratory evidence
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7. Triads & Natural History
Person
Age
Gender
Marital status
Ethnicity/Race
Behavior / life-style factors
Socio-economic status
Education
Occupation
Income
Biological factors, passive/active immunity, concomitant illness
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8. Triads & Natural History
Place
Geographically restricted or widespread (pandemic)?
Relation to water or food supply (clusters: multiple / one)
Residence (rural, urban, remote)
Climate (temperature, humidity)
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9. Triads & Natural History
Time
Changing or stable?
Seasonal variation.
Clustered (epidemic) or evenly distributed (endemic)?
Point source or propagated.
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10. Triads & Natural History
Time Trends
Point source e.g. food-borne outbreaks), in terms of hours / days
Seasonal - cyclicity (e.g. common cold, influenza), in terms of months
Propogative (e.g. water borne epidemics), in terms of weeks / months
Secular (e.g. morbidity / mortality of non-communicable diseases), in terms of years
Cluster in time / place
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11. The Basic Triad Of Analytic Epidemiology
THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE:
HOST
Define host, agent?
AGENT
ENVIRONMENT

12. The Analytical Epidemiologic Triad
This model comprises a susceptible host (the person at risk for the disease), a disease agent (the proximate cause), and an environmental context for the interaction between host and agent.
Thus, development of disease is a combination of events:
A harmful agent
A susceptible host
An appropriate environment
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13. Triads & Natural History
Agents
Biological (micro-organisms)
Physical (temperature, radiation, trauma, others)
Chemical (acids, alkalis, poisons, tobacco, medications / drugs, others)
Environmental (nutrients in diet, allergens, others)
Nutritional (under- or over-nutrition)
Psychological experiences
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14. Triads & Natural History
Host Factors
Host factors are intrinsic factors that influence an individual’s exposure, susceptibility, or response to a causative agent. These include:
Genetic endowment
Immunologic state
Personal behavior (life-style factors): diet, tobacco use, exercise, etc
Personal characteristics (described before, under “person”), including: age, gender, socio-economic status, etc.
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15. Triads & Natural History
Environment
Environmental factors are extrinsic factors which affect the agent and the opportunity for exposure. These include:
Physical factors: e.g. geology, climate (temperature, humidity, rain, etc)
Biological factors: e.g. insects that transmit an agent
Socioeconomic factors: e.g. crowding, sanitation, and the availability of health services
Phenomena which bring the host and agent together: vector, vehicle, reservoir, etc
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16. Triads & Natural History
Summary of Analytical Triad
Agent factors include infectious microorganisms, e.g. virus, bacterium, parasite, or other agents.
They may be necessary but not always sufficient alone to cause disease.
Host factors are intrinsic factors that influence an individual’s exposure, susceptibility, or response to a causative agent
Environmental factors are extrinsic factors which affect the agent and the opportunity for exposure. 16
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17. Triads & Natural History
Example
The number of people who become diseased with tuberculosis will depend on:
characteristics of the agent,
environmental factors,
And host factors
Explain some of these factors
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18. The Analytical Epidemiology Triad
Host:
Intrinsic factors, genetic, physiologic factors, psychological factors, immunity
Health or
Illness ?
Agent:
Amount, infectivity, pathogenicity, virulence,….
Environment:
Physical, biological, social 18
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19. Natural History of Disease

20. Natural history of disease
Natural history of disease refers to the progress of a disease process in an individual over time, in the absence of intervention.
The natural history of a disease describes the course of the disease in an individual starting from the moment of exposure to the causal agents till one of the possible outcomes occurs. 20 20
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21. Natural history Phenomena
Induction : time to disease initiation
Incubation:– time to symptoms (infectious disease)
Latency: time to detection (for non- infectious disease) or to infectiousness
Several important landmarks in natural history are the initiation of the disease itself (however we are defining it), the onset of symptoms, the point where a disease can be and/or is usually detected, and for communicable diseases, the point when the disease becomes transmissible.
Epidemiology is still working on standardizing its terminology. The terms presented here are used by a major epidemiology textbook (Rothman and Greenland, Modern Epidemiology) to represent these four stages:
Rothman applies the term induction period to the period of time between exposure to a causal agent and the initiation of the disease. Since the etiology of many diseases involves a combination of factors, Rothman considers each causal agent as having its own induction period. The beginning of actual disease, even if it can be defined, is often unobservable. Nevertheless, the concept of the induction period is a useful one.
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22. Natural history of disease22 22
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23. Natural history of disease
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Natural history of disease 23
Dr. Salwa Tayel 23

24. Natural History of Disease
Detectable subclinical disease
Onset of symptoms
Susceptible Host
Point of Exposure
Subclinical Disease
Clinical Disease
Outcome:
Stage of Recovery, Complications, Disability, or Death
Screening
Diagnosis sought
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25. Triads & Natural History
The problem
The problem is that we might know about disease onset when symptoms occur but most likely we will only know about the disease when a person seeks care for the symptoms.
In some situations an investigator will only become aware of a case after a diagnosis is made.
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26. Importance of studying Natural history of disease
The understanding of this progression from disease onset to cure or death is important for epidemiologists.
Natural history is as important as causal understanding for the prevention and control of disease.
The earlier you can become aware of the attack the more likely you will be able to intervene and save lives.
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27. Triads & Natural History
Reference books
Principles of Epidemiology in Public Health Practice. Third Edition. An Introduction to Applied Epidemiology and Biostatistics. Centers for Disease Control and Prevention (CDC)
Gordis L. Epidemiology. 2009
Triads & Natural History