This document provides an overview of the natural history of disease and levels of disease prevention. It discusses the natural history of disease in four stages: susceptibility, pre-symptomatic disease, clinical disease, and disability/death. It also defines different levels of disease occurrence such as endemic, epidemic, outbreak, and pandemic. Finally, it outlines the four main levels of disease prevention: primordial, primary, secondary, and tertiary prevention. The goal of epidemiology is to understand disease causation and distribution in order to implement effective prevention and control measures at various levels.

1. COURSE TITLE: INTRODUCTION TO
EPIDEMIOLOGY
FOR BSc NURS STUDENTS
BY: Agumas Fentahun
(BSC in HO, MPH in Epidemiology)
1

2. Introduction to Epidemiology
 Definition
 Basic features of Epidemiology
 Branches of epidemiology
 History of Epidemiology
 Scope of epidemiology
 Use/purpose/applications/ of Epidemiology
 Basic assumptions of epidemiology
 Theories disease causation
 Natural history of disease
 Levels of disease occurrence & level of
2

3. Introduction: Definition
 The term ‘epidemiology’ comes from the Greek
words epi(upon), demos(people) and
logos(the study of). i.e. Epidemiology is the
study of what is ‘upon the people’
 Epidemiology is classically defined as: ‘the
study of the distribution, determinants, and
frequency of diseases & health-related
problems in human populations’ and the
application of this study to the prevention of
disease and promotion of health
3

4. Definition…
Key terms
 Distribution: Epidemiology is concerned with
the frequency and pattern of health events in a
population.
 Frequency: involves quantification of occurrence of
disease or other health related problems
 Pattern: occurrence of disease or health-related events
by time, place, and person
 Time patterns (annual, seasonal, weekly, etc)
 Place patterns (geographic variation, urban/rural
differences, location of work sites, etc)
 Personal characteristics (demographic factors such as
age, sex, marital status, and socioeconomic status)
4

5. Definition…
 Determinants: factors that influence or
determine the occurrence of disease & other
health-related events
 any definable entity that brings about change in
health and health related conditions
 It refers to “why diseases occur in certain places,
in a certain period, Or in a certain population
groups”
 What makes some people healthy and others
unhealthy?
 The range of personal, social, economic, and
environmental factors that influence health status
5

6. Definition…
 Health-related states or events:-anything that
affects the well-being of a population, including
malnutrition, injuries, etc
 Study:
 Epidemiology is a scientific discipline with sound
methods of scientific inquiry at its foundation.
 Epidemiology is data-driven and relies on a systematic
and unbiased approach to the collection, analysis, and
interpretation of data.
6

7. Definition…
 Application: involves applying the knowledge
gained by the studies to community-based
practice
 The ultimate purpose of all epidemiological
studies is the prevention and control of health
problems
7

8. Definition…
 Risk factor: an individual attribute (intrinsic
characteristic of the individual) or exposure (external
environmental situation) that is associated with
(increases the likelihood of) the occurrence of a
disease
 Necessary cause: factor necessary for a disease to
occur and in its absence that specific disease can’t
occur.
8

9. 9
Basic features of Epidemiology
1. Studies are conducted on human population
2. It examines patterns of events in people
3. Can establish cause-effect relationship
without the knowledge of biological
mechanism
4. It covers a wide range of conditions
5. It is an advancing science
Agumas F.

10. Types of Epidemiology
Two major categories of Epidemiology
1. Descriptive Epidemiology
Defines distribution (frequency and
patterns) of diseases and other health
related events
Answers the four major questions: how
many, who, where, and when?
Agumas F.
10

11. Types…
2. Analytic Epidemiology
Analyses determinants of health problems
Answers two other major questions: how?
and why?
Generally, Epidemiology answers six major
questions: how many, who, where, when, how
and why?
Agumas F.
11

12. Question 1
 What are personal, social, economic, and
environmental determinants of disease?
12

13. History of Epidemiology
Seven land marks in the history of
Epidemiology
1. Hippocrates (460BC): Environment &
human behaviors affects health
2. John Graunt (1662): Quantified births,
deaths and diseases
3. Lind (1747): Scurvy could be treated with
fresh fruit
4. William Farr (1839): Established
application of vital statistics for the
evaluation of health problems
Agumas F.
13

14. History…
5. John Snow (1854): tested a hypothesis on the
origin of epidemic of cholera
6. Alexander Louis (1872): Systematized
application of numerical thinking (quantitative
reasoning)
7. Bradford Hill (1937): Suggested criteria for
establishing causation
Epidemiological thought emerged in 460 BC
Epidemiology flourished as a discipline in
1940s
Agumas F.
14

15. Scope of Epidemiology
Originally, Epidemiology was concerned with
investigation & management of epidemics of
communicable diseases
Lately, Epidemiology was extended to
endemic communicable diseases and non-
communicable infectious diseases
Recently, Epidemiology can be applied to all
diseases and other health related events
Agumas F.
15

16. Scope of epidemiology
 In general Currently, Epidemiology Used in
all kinds of diseases-communicable, non-
communicable or injury category, in laboratory
sciences, clinical medicine and public health
 Ranges from routine surveillance to research
strategies for the testing of hypotheses
16

17. Purposes of epidemiology
The ultimate purpose of Epidemiology is
prevention of diseases and promotion of health
How?
 Establish causation of disease
 Elucidate the natural history of disease
 Describe the health status of the population
 Identify determinants of diseases
 Evaluation of intervention programs
 Classification of diseases
17

18. Purposes…
1. Establish causation of disease
 Causation-is an antecedent factor that was
necessary for the occurrence of the disease at
the moment it occurred
2. Studies the natural history of diseases
 is essential to make (study) prognosis or the
likely outcome of a patients illness
 Gives basis for rational decision about therapy
18

19. Purposes…
3. Description of health status of
populations:
 Measure disease frequency- quantify disease
 Assess distribution of disease
 Who is getting disease?
 Where is disease occurring?
 When is disease occurring?
 Formulation of hypotheses concerning causal
 and preventive factors
19

20. Purposes…
4. Identify determinants of diseases:
 Hypothesis is developed after description of
occurrence by person, place and time.
 Hypotheses are tested using epidemiologic
studies
 =>Determinants:
 lifestyle, poverty, health care facilities,
environment
 If we can identify determinants of disease,
lead or helps to prevention and control of
20

21. Purposes…
5. Evaluation of intervention programs:
 assessing whether the measures
(programs designed to prevent and control
a disease) are effective in reducing the
frequency of the d/se or not
6. Classification of diseases:
 e.g. based on route of transmission
21

22. Purposes…
 Is epidemiology of importance to
environmental health professionals?
 In what aspects?
22

23. Basic Epidemiologic
Assumptions
Two fundamental assumptions:
 Human disease does not occur at random:
 some behavioral and environmental factors
(exposures) increase the risk of
acquiring/developing a particular disease among
group of individuals
 Human disease has causal and preventive
factors
 identified through systematic investigation of
populations, creating opportunity for prevention
and control the diseases, either by eliminating the
cause or introducing appropriate treatment
23

24. Concepts of Disease Occurrence (causation)
 A critical premise of epidemiology is that
diseases do not occur randomly in a
population,
 are more likely to occur in some members of the
population than others b/se of risk factors that
may not be distributed randomly
 Causation
 A number of models of disease causation have
been proposed,
 The simplest of these is the epidemiologic triad
or triangle, the traditional model for infectious
disease
24

25. Concepts of Disease
Occurrence…
 Epidemiologic triad is a model of disease causation
famous in epidemiology
 The triad consists of an external agent, a
susceptible host, and an environment that brings the
host and agent together
 In this model, disease results from the interaction b/n
the agent and the susceptible host in an environment
that supports transmission of the agent from a source
to that host
25

26. Concepts of Disease
Occurrence…
 Agent, host, and environmental factors
interrelate in a variety of complex ways to
produce disease
 Development of public health measures to
control or prevent disease usually requires
assessment of all three components and their
interactions
26

27. Concepts of Disease
Occurrence…
 Figure :1 Epidemiologic Triad
27

28. Concepts of Disease
Occurrence…
Agent
 originally referred to an infectious
microorganism or pathogen: a virus,
bacterium, parasite, or other microbe
 Generally, the agent must be present for
disease to occur; however, presence of that
agent alone is not always sufficient to cause
disease
 A variety of factors influence whether exposure
to an organism will result in disease, including
the organism's pathogen city (ability to cause
28

29. Concepts of Disease
Occurrence…
 Over time, the concept of agent has been
broadened to include chemical and physical
causes of disease or injury
 While the epidemiologic triad serves as a
useful model for many diseases, it has proven
inadequate for cardiovascular disease, cancer,
and other diseases that appear to have
multiple contributing causes without a single
necessary one.
29

30. Concepts of Disease
Occurrence…
Host
 Refers to the human who can get the disease
 A variety of factors intrinsic to the host,
sometimes called risk factors, can influence an
individual's exposure, susceptibility, or
response to a causative agent
 Opportunities for exposure are often
influenced by behaviors such as sexual
practices, hygiene, and other personal choices
as well as by age and sex.
30

31. Concepts of Disease
Occurrence…
Environment
 Refers to extrinsic factors that affect the agent
and the opportunity for exposure
 Environmental factors include:
 physical factors such as geology and climate,
 biologic factors such as insects that transmit the
agent, and
 socioeconomic factors such as crowding,
sanitation, and the availability of health services
31

32. Examples of causes of disease by host, agent and
environmental factors.
Host factors Agent factors Environmental
factors
Age
 Sex
 Previous
disability
 Behaviors
 Genetic
inheritance
 Height
 Weight
Virulence of
organisms
 Serotype of
organisms
 Antibiotic
resistance
Cigarette-tar
content
Home
overcrowding
 Air pollution
 Workplace
Hygiene
 Weather
 Water
composition
 Food
contamination
 Animal contact
1/26/2023
By: Agumas F.

33. Reading Assignment
1. What does mean causation?
2. Necessary vs. sufficient cause
3. Theories of disease causation?
4. How do diseases develop (Disease causation
models (concepts))?
4.1. Epidemiologic triad
4.2. Web causation??
4.3. wheel causation??
33

34. 34
Agumas F.

35. 35
1/26/2023 Agumas F

36. COURSE TITLE: Natural History of Disease and
Level of Disease Prevention
BY: Agumas Fentahun
(BSC in HO, MPH in Epidemiology)
36

37. Natural History of Disease and Level of
Disease Prevention
37

38. Natural history of disease
• refers to the progression of disease process in an
individual over time, in the absence of intervention
• It is the course of illness in the absence of intervention
• begins with exposure to the causative agent
• ends with recovery, disability, or death
• Has four stages
– Stage of susceptibility
– Stage of pre-symptomatic (sub-clinical) disease
– Stage of clinical disease
– Stage of disability or death
38

39. Natural history of disease…
1. Stage of susceptibility
• In w/c, disease has not yet developed, but the
groundwork has been laid by the presence of
factors that favor its occurrence
– Example: unvaccinated child is susceptible to measles
2. Stage of Pre-symptomatic (sub-clinical) disease
• no manifestations of the disease but pathologic
changes have started to occur in the body
• detected through special tests
• may lead to the clinical stage, or ends in recovery
39

40. Natural history of disease…
• Examples:
– Detection of antibodies against HIV in an
apparently healthy person
– Ova of intestinal parasite in the stool of apparently
healthy children
40

41. Natural history of disease…
3. The Clinical stage
• Characterized by signs and symptoms of the
disease
• varies in duration, severity and outcome for
d/t diseases
• May end with recovery, disability or death
41

42. Natural history of disease…
Examples:
• Common cold: has a short and mild clinical stage
and almost everyone recovers quickly
• Polio: has a severe clinical stage and many
patients develop paralysis
• Rabies: has a relatively short but severe clinical
stage and almost always results in death
• Diabetes Mellitus: has a relatively longer clinical
stage and eventually results in death if the
patient is not properly treated
42

43. Natural history of disease…
4. Stage of disability, death or recovery
where the out come of the disease observed
• Disability is a physical or mental condition that
limits a person's movements, senses, or
activities.
• Examples:
– Trachoma may cause blindness
– Meningitis may result in blindness or deafness, or
death
43

44. Fig1: Natural History of Disease Timeline
44
Natural history of disease…

45. Level of disease occurrences
Level of disease
• Endemic: a persistently low to moderate level
of occurrence of disease
• Epidemic: presence of disease in excess of the
usual occurrence
• Outbreak: carries the same definition of
epidemic, but is often used for a more limited
geographic area. It is an epidemic restricted by
place, person and time
45

46. Level of disease…
• Pandemic-an epidemic that has spread over
several countries or continents
• Hyper-endemic: a persistently high level of
occurrence of a disease
• Sporadic: refers to a disease that occurs
infrequently and irregularly.
• Cluster: refers to an aggregation of cases grouped
in place and time that are suspected to be greater
than the number expected, even though the
expected number may not be known.
46

47. Level of disease…
An epidemic may result from:
– A recent increase in amount or virulence of the agent
– The recent introduction of the agent into a setting
where it has not been before,
– An enhanced mode of transmission so that more
susceptible persons are exposed,
– A change in the susceptibility of the host response to
the agent, and/or
– Factors that increase host exposure or involve
introduction through new portals of entry
47

48. Levels of Disease Prevention
• The major purpose in investigating the
epidemiology of diseases is to learn how to
prevent and control them
• Disease prevention means to interrupt or slow
the progression of disease
48

49. Levels of Disease prevention
Primordial prevention
Primary prevention
Secondary prevention
Tertiary prevention
1/26/2023 By: Agumas F.

50. Eradication and Elimination
Eradication:
• It is termination of all transmission of infection
by extinction of the infectious agent through
surveillance. it is an absolute process, an “all or
none” phenomenon, restricted to termination of
infection from the whole world.
Elimination:
• Is sometimes used to describe eradication of a
disease from a large geographic region.
• Disease which are amenable to elimination in
the meantime are polio, measles and diphtheria.
1/26/2023 By: Agumas F.

51. Level Of disease Prevention
1. Primordial prevention
• Primordial prevention consists of actions and
measures that inhibit the emergence of risk
factors in the form of
– environmental,
– economic,
– social,
– behavioral conditions and cultural patterns of living etc.
1/26/2023 By: Agumas F.

52. Level of Disease prevention
2. Primary prevention
• Any action that are taken prior to the onset of
disease, which removes the possibility that
the disease will ever occur.
• It signifies the intervention in the pre-
pathogenesis phase of a disease.
1/26/2023 By: Agumas F.

53. Levels of Disease Prevention…
2. Primary prevention:-keeps the disease
process from becoming established by
eliminating causes of disease or increasing
resistance to disease
• Has 3 components
– These are health promotion, prevention of
exposure, and prevention of disease
53

54. Levels of Disease Prevention…
2.1. Health promotion:- consists of general non-
specific interventions that enhance health and
the body's ability to resist disease
– Improvement of socioeconomic status, provision of
adequate food, housing, clothing, and education are
examples of health promotion
2.2. Prevention of exposure:- is the avoidance of factors
which may cause disease if an individual is exposed to
them
– Examples can be provision of safe and adequate
water, proper excreta disposal, and vector control.
54

55. Levels of Disease Prevention…
2.3. Prevention of disease:- is the prevention of
disease development after the individual has
become exposed to the disease causing factors
– Immunization is an example of prevention of disease
– it acts after exposure has taken place
– It does not prevent an infectious organism from
invading the immunized host, but does prevent it from
establishing an infection
– eg. measles vaccine, will not prevent the virus from
entering to the body but it prevents the development
of infection/disease
55

56. Levels of Disease Prevention…
3. Secondary prevention:-is to stop or slow the
progression of disease so as to prevent or
limit permanent damage
– can be achieved through detecting people who
already have the disease as early as possible and
treat them
– It is carried out before the person is permanently
damaged
• Eg diagnosing and treatment of a certain
individual
56

57. Levels of Disease Prevention…
Examples:
• Prevention of blindness from Trachoma
• Early detection and treatment of breast Ca to
prevent its progression to the invasive stage
57

58. Level of Disease prevention
4. Tertiary prevention:
Is targeted towards people with permanent damage or
disability
If primary and secondary preventions have failed, and
When primary and secondary prevention are not
effective
58

59. Levels of Disease Prevention…
• It has two objectives:
1. Treatment to prevent further disability or death and
2. To limit the physical, psychological, social, and
financial impact of disability-improving the quality
of life
This can be done through rehabilitation, which is the
retraining of the remaining functions for maximal
effectiveness
Example: in blindness due to vitamin A deficiency,
tertiary prevention (rehabilitation) can help the blind
or partly blind person learn to do gainful work and be
economically self supporting
59

60. Thank you!
1/26/2023 Getasew M. 60