2. 1. DEFINITION OF EPIDEMIOLOGY
• Originated from three Greek words.
on, up on People doctrine, study
• The term epidemiology was originally
applied to the study of acute infectious
diseases and was defined as:
EPI demos Logus
2
3. Definition Cont….
• “the science of epidemics.”
• “The study of the distribution and
dynamics of disease in the human
population.”
• “The study of the distribution and
determinants of health related
phenomena in the human population.”
3
4. Current definition
“The study of the frequency, distribution
and determinants of diseases and other
health related problems in human
population and the application of this to the
prevention and control of health problems.”
4
5. Terms taken from the definition
1. Frequency: It is a quantitative science
(it measures frequency of occurrence)
• It assesses occurrence of a diseases in
numbers
5
6. Cont…
2. Distribution – is the occurrence of
diseases and other health problems in
terms of person, place and time.
• This attempts to answer
Who? Where? and When?
of disease occurrence.
6
7. Cont…
3. Determinants – are factors that are related
with the occurrence of a disease in
individuals,
– It is about causative factors for the
occurrence of a disease.
– It attempts to answer questions like
How? and why? disease occurs?
7
8. Cont…
4. Disease and Health-related problems
It includes – both diseases and physiological
and/or psychological dysfunction of the body
It also includes other health related problems
like:
– Vital events, births, deaths, marriage, divorces etc
– Health related behaviour – sexual behaviour,
smoking, alcoholism, drug abuses
– Social factors, such as poverty.
8
9. Disease Classification
Diseases can be classified According to
two dimensions, time course and cause
A. Time course:
– Acute disease –
• Characterized by a rapid onset and short
duration.
E.g cholera, influenza, malaria etc
– Chronic disease –
• Characterized by a prolonged duration.
E.g. TB, leprosy, AIDS etc
9
10. Cont….
B. Cause:
– Infectious – caused by living organisms
such as: virus, bacteria, parasites, etc.
– Non-infectious– caused by something
other than living organism.
• E.g. Goiter, hypertension, diabetes, etc
10
11. Cont…
5. Human population –
Epidemiology studies the health of the
groups, communities and whole population.
(Specific population)
Clinical medicine – focuses largely on the
health of sick individuals.
11
12. Cont…
6. Application …. to the prevention and
control of …..-
It is applied science, ie direct practical
applications.
The aim of all about frequency, distribution
of diseases is to identify effective
prevention and control strategies.
12
13. 2. SCOPE OF EPIDEMIOLOGY
• Historically, epidemiology was largely
concerned with infectious diseases and
mainly epidemics.
• Now, it has expanded to include both
infectious and non-infectious diseases.
13
14. Cont….
• Epidemiology was later applied to:
– Malnutrition
• PEM
• Vitamin deficiencies (Vit A, B, C, D)
• Goiter
– Malignancies
• Cigarette smocking and cancer
• Certain chemicals and cancer
14
15. Cont….
• Cardiovascular diseases
– Change in life style, obesity
– Stress, lesser physical exercises
– High intake of calories and saturated fats
• Chronic diseases
– Rheumatoid arthritis
– Mental illness
– Diabetics, etc
15
16. 3. PRIMARY USES OF EPIDEMIOLOGY
A. Population or community health assessment
Eg
– Could measure availability, accessibility and
effectiveness of health service
– Could assess availability of diseases, etc
B. Identification of population at risk
Eg
Youth – high risk for HIV/AIDS
Early marriage - risk for cancer of cervix or uterus
Urbanization- increase in prevalence of HIV/ AIDS
16
17. Cont…
C. Individual decisions
People may not realize they are using
epidemiology on daily decisions,
Eg The effectiveness of health education
on behavioral changes of the
population towards,
• HIV/ AIDS transmission,
• Cleanliness,
• Taking prophylaxis etc
17
18. Cont…
D. Completing the clinical picture of a
disease
Example.
– HIV/AIDS,
– Malaria
– Depression etc
18
19. 4.CAUSAL/ PREDISPOSING FACTORS FOR
DISEASE
Factors affecting the development of disease can
be divided into three groups:
Agent, Host Environment.
1. Agent - is synonymous with the primary/ true
cause without which a specific disease cannot
occur.
2. Host - refers to human beings or group of
population of immediate concern.
3. Environment - includes all external area
outside the agent and the human host
19
24. II. Host factors (Intrinsic factors)
The state of the host at any given time is the
result of the interactions of genetic
endowment over the entire lifespan.
1. Inherent characteristics:
-Genetic- haemophilic, diabetes, colour
blindness, asthma
-Age - child hood diseases
-Neoplastic diseases increase with age
24
26. Cont…
2. Acquired characteristics:
– Immune status
– Marital status - life style difference
– Nutritional status
3. Activity related:
– Work
– Exercise
– Recreation
– Religious practices
– Customs, etc.
26
27. Cont….
4. Related to living conditions:
– Social
– Economic
– Environmental, etc.
27
28. III. Environmental factors
(Extrinsic factors)
Very numerous, Commonly
subdivided in to three classes which
relate, respectively to:
1. The biologic environment
2. Physical environment
3. Socio-economic components of the
environment
28
29. 1) Biological environment
This sector of the environment includes:
– Reservoirs of infection
– Vectors that transmit disease (e.g., flies
and mosquitoes),
– Plants and animals (as sources of food,
shelter for agents).
29
30. Cont….
Climate and ecological characteristics determine
the animal and vegetation environment
a) Temperature, humidity and other conditions
appropriate for the survival of parasites
outside the body
eg. Cercaria of schistosoma,
Larval stages of some hookworm
b) Conditions suitable for the reproduction
of animal vectors
eg. Mosquitoes, sand flies, tse-tse flies, ticks, louse etc
30
31. 2) Physical environment
The physical aspects of the environment
include:
Heat, Light, Air, Water,
Atmospheric pressure,
Humidity
Chemical agents of all kinds,
Ionization (Radiation) etc.
31
32. Cont….
In the technically developed areas of the
world, humans have a great deal of control
over the physical environment through:
– Provision of adequate shelter against extremes of
weather,
– Purification of drinking water,
– Treatment of sewerage, and
– Year-round control of indoor temperature and
humidity.
32
33. Cont…
New environmental problems continue to arise as
old ones are solved. Currently,
• The rapid growth of population,
• The increase in industrial wastes of all kinds,
• The ever-increasing number of motor vehicles
interact to:
– produce air, water, noise, and other types of pollution of the
environment.
– Air pollution, for example, has recently emerged as an
urgent threat to health. (penetration of O3)
33
34. 3) Social and economical environment
The social environment may be defined in terms of the
overall economic, social and political organization of
a society and of the institutions
• Overall socio-economic and political organization
affect
– Support for medical care and biomedical research,
– The adequacy and level of enforcement of codes and laws
controlling health related environmental hazards (pollution,
housing, occupational safety, and so on).
• Although three discrete sectors of the environment
(biological, social, and physical) have been
identified, it is difficult to differentiate them.
34
35. Interrelations of factors
(Ecologic Models):
• It is the interaction of these three sets of factors, which
determines whether a disease develops or not.
• Several alternative models have been developed to
depict the way in which these interactions influence the
occurrence of disease.
• Whichever model one uses, it is important to realise
that the balance of forces that determines an
individual’s state of health at a given time is in a kind of
dynamic equilibrium.
• A potentially harmful change in any of the components
of the system may not lead to detectable disease if the
other parts of the system have the capacity to
compensate. 35
36. Three ecological models
1. The Epidemiologic Triangle
--The epidemiologic triangle
is considered to consist of
three components – host,
environment and agent.
– The model implies that each must be
analysed and understood for
comprehension and predictions of patterns
of a disease.
– A change in any of the components will
alter an existing equilibrium to increase or
decrease the frequency of the disease.
36
37. II. The Web of Causation
• The essence of the concept is that effects (diseases)
never depend on single isolated causes, but rather
develop as a result of chains of inter-related causes.
• The large number of inter-related causes create a
condition that may appropriately be conceptualized as a
“web”, which in its complexity and origins lies quite
beyond our understandings.
Eg Pulmonary Tuberculosis-
• Patients lower immunity + M. Tbc
• Malnutrition, HIV/AIDS, Age, Cancer
37
40. III. The Wheel / Pie Model
• This has come for multi-factorial nature of causation
in many diseases.
• Component, sufficient and necessary causes.
Sufficient Cause:
• As it is in the figure, the whole components of a pie
make the sufficient cause for a disease.
• A disease may have more than one sufficient cause,
each sufficient causes are composed of several
component causes.
• A disease may have only one factor as a sufficient
cause to develop a disease. Eg. Rabies 40
41. Cont…
Contributing Cause:
• Each factor (pieces in the pie) that
contributes to causation of a disease is
called contributing cause.
Necessary Cause:
• A factor (contributing) that is necessary (or with out
which) the disease doesn’t exist or occur is a necessary
cause
• Knowledge on all contributing factors is not necessary
for a prevention of a disease. By eliminating only one
contributing cause from a sufficient cause could
prevent the disease caused by such sufficient cause 41
42. Fig I. Conceptual cause of a certain hypothetical disease.
Sufficient cause I Sufficient cause II Sufficient cause III
C
D
A
B
F
G
A
E
I
J
A
H
‘A’ is a necessary cause of the hypothetical disease.
A, B, C, D are sufficient cause of disease I
‘A’ is a component & necessary cause of the
hypothetical disease
‘B’ is only a component cause of disease I 42
44. MECHANISMS OF DISEASE TRANSMISSION
• Infectious diseases still account for
most of the morbidity and mortality in
developing countries.
• This unit presents some of the
important terms and concepts needed
to understand the epidemiology of
infectious diseases.
44
45. The Infectious Process:
Traditionally the infectious process for a
specific disease is described in terms of the
following six components:
1. The agent
2. Its reservoir (s)
3. Its portal (s) of exit
4. Its mode (s) of transmission
5. Its portal (s) of entry
6. The human host
45
46. Cont…
• These six components put together and
constitute the chain of transmission or
transmission cycle.
• Possible portals of exit for an infectious agent
include:
A. All body secretions and discharges (mucus,
saliva, tears, breast milk, vaginal, cervical, and
urethral secretions, semen, pus, exudates from
wounds)
B. Excretions (faeces and urine), blood, tissues
(including the placenta).
46
47. Cont…
• The manner of entry of an infectious
agent into a host is one of the factors,
which determines whether or not the
agent will succeed in establishing an
infection.
47
49. 2. Reservoir
• This component of an infectious process
is defined as an organism or habitat in
which an infectious agent normally lives,
transforms, develops and/or multiplies.
• Thus, reservoirs of infection include
human beings, vertebrate animals, and
environmental sources (plants, soil,
water, etc).
• Many infectious diseases have more than
one reservoir. (eg Rabies, Anthrax, etc)
49
50. Cont…
1. Humans as source of infectious agents
(two forms).
a. Persons with symptomatic illnesses
This people are already sick and are able
to transmit the agent.
But they are less likely to transmit
infection widely because of their
symptoms increases their likelihood of
getting diagnosis and treatment.
50
51. Cont…
b. Carriers
Carrier are persons without apparent disease,
but who are able to transmit an agent of a
disease.
Carriers may be:
– Asymptomatic (Never show symptoms) eg HIV/ AIDS
– Incubatory (before becoming clinically ill) eg measles
– Convalescent (After symptoms disappear) eg measles
– Chronic (after they are cured, they harbour an agent)
51
52. Fig 2. Time course of a disease in relation to
its clinical expression and communicability
Time of infection
(biological onset)
Agent starts
being shed
1st manifestation of
disease (clinical onset)
Recovery Agent stops
being shed
Relapse
TIME
Incubation period
Latent period
Prepatent
period
Communication period
Generation
Clinical
Threshold
Asymptomatic
Symptomatic
Clinical case
Asymptomatic carrier
Chronic
carrier
Incubatory
transmission
Convalescent
transmission
52
53. Cont….
• Carriers transmit a disease at a higher rate
because they do not recognise that they are
infected.
• They will not take any precaution of not
transmitting.
• Eg many carrier of HIV/ AIDS.
carriers of Salmonella (Typhoid fever)
53
54. Cont…
2. Animals as source of infectious agents
• Infectious diseases that are transmitted
from animals to human being are called
zoonoses.
• Examples
– Brucellosis (cows and pigs)
– Anthrax (sheep)
– Plague (Rodents)
– Rabies (Bats, Dogs and other animals)
– Taeniasis (cows) etc. 54
55. Cont…
3. Environment as a source of infectious
agents
– Plants, soil and water in the environment
are also reservoirs for some agents.
– Many fungal agents, including parasitic
infestations grow and complete their
growth in soil and water.
– Eg Hookworm, Ascaries, Schistosomiasis
55
56. 3. Portals of exit
Portal of exit is the path by which an agent
leaves or escapes the host.
The portal of exit usually corresponds to the
site at which the agent is localised.
a) Respiratory (upper, lower) eg. - diphtheria -
TBC.
b) Alimentary - typhoid fever, amoebiasis, etc.
c) Geneto-urinary – STDS, HIV/AIDS.
d) Skin - scabies
e) Trans-placental - HIV, Malaria, Syphilis
56
57. 4. Modes of transmission
1. Direct transmission –
• direct transfer of etiologic agent from
host or reservoir to susceptible person.
A. Direct contact spread - kissing, sexual
transmission, faeces-hand-mouth in
shigellosis, skin-to-skin, biting (rabies),
etc.
57
58. Cont….
B. Droplet (Direct projection) spread -
coughing, sneezing, spitting, talking,
etc.
C. Trans-placental transmission – a
special form of direct transmission
from mother to foetus through the
placenta as in syphilis, HIV/AIDS.
58
59. Cont…
2. Indirect transmission –
• Agent spread from reservoir or host
suspended in the air are called
airborne, by inanimate are called
Vehicle or Vehicle-borne and by
animate are called Vector or vector-
borne.
A. Vehicle (Vehicle-borne) -
Food, utensiles, clothes, needles,
surgical instruments, etc
59
60. Cont…
B. Vector
I- Mechanical
At this stage the agent will not grow or
multiply in the vector
eg- flies
II Biological
At this stage the agent will grow and/or
multiply in the vector, (ie the vector is
serving both as an intermediate reservoir
and a mode of transmitter).
eg - mosquitoes, sand-flies, tse-tse fly
60
61. Cont…
c) Airborne (dried residue of droplet nuclei, dust)
eg Tuberculosis.
• A disease often may have several modes of
transmission.
• For example, trachoma can be transmitted
directly from one person to another, or through
a vehicle such as a contaminated towel, or
through a vector (flies).
61
62. 5. Portal of entry in human host
• An agent enters a susceptible host through a
portal of entry.
• Portal of entry must give access to the agent
to multiply and act.
• Agents usually use the same portal to enter a
new host that they use to exit the source
host.
Eg. Intestinal parasites.
Bacterial Pneumonia
62
63. 6. Susceptible host
1. Genetic factors – An individual’s
genetic make up either may increase
or decrease susceptibility.
• This is due to difference in the skin,
mucouse membrane, gastric acidity,
cilia in the respiratory tract, the cough
reflex etc, of individuals.
63
65. TIME COURSE OF AN INFECTIOUS
DISEASE
Pre-patent period
– The time interval between infection (biological
onset), and the point at which the infection can be
first detected
Incubation period –
– The time interval between infection and the first
clinical manifestations of disease, i.e. between
biological onset and clinical onset.
Latent period
– The time interval between recovery and the
occurrence of relapse or recrudescence in clinical
disease.
65
66. Fig 2. Time course of a disease in relation to
its clinical expression and communicability
Time of infection
(biological onset)
Agent starts
being shed
1st manifestation of
disease (clinical onset)
Recovery Agent stops
being shed
Relapse
TIME
Incubation period
Latent period
Prepatent
period
Communication period
Generation
Clinical
Threshold
Asymptomatic
Symptomatic
Clinical case
Asymptomatic carrier
Chronic
carrier
66
67. Natural History of diseases
• The natural history of a disease refers to the
progress of a disease process in an individual
over time, in the absence of intervention.
• It begins with an exposure of a susceptible
host to a causative agent.
• With out medical intervention, the process
ends with recovery, disability or death.
• Usual course of a disease may be halted at any
point in the progression by preventive and
therapeutic measures.
67
68. Natural History of diseases
Usual Time
Of diagnosis
Exposure Pathologic Onset of
changes symptoms
Stage of
Susceptibility
Stage of
sub-clinical disease
Stage of
Clinical disease
Stage of recovery, disability
or death
68
69. Level of prevention
There are several stages during the course of a
disease at which we can intervene in order
to control the disease.
Three levels, (Primary, secondary and tertiary)
A. Primary prevention
The objectives here are to promote health,
prevent exposure, and prevent disease.
69
70. Cont…
1. Health promotion:
• This consists of general non-specific
interventions that enhance health and the
body’s ability to resist disease – including:
• The improvement of socioeconomic status
through the provision of adequately.
– paid jobs,
– education,
– affordable and adequate housing and clothing,
etc.
70
71. Cont…
2. Prevention of exposure:
• There are many examples of interventions
aimed at this stage, such as
– the provision of safe and adequate
water, of proper excreta disposal,
– Provision of vector control;
71
72. Cont…
–Provision of a safe environment at
home
–Example;
- proper storage of insecticides and
medicines, out of children’s reach),
- on the streets (e.g., driver licensing
laws).
72
73. Cont…
3. Prevention of disease:
– An example of intervention, which
acts at this stage, is immunization.
– Breastfeeding is an example of an
intervention which acts at all three
levels of primary prevention.
73
74. Cont…
B. Secondary prevention
– After the biological onset of disease, but
before permanent damage sets in , we
speak of secondary prevention.
– The objective here is to stop or slow the
progression of disease so as to prevent
or limit permanent damage.
– Strategy at this stage is through early
detection and treatment of disease.
74
75. Cont…
c. Tertiary prevention
• After permanent damage has set in,
the objective of tertiary prevention is
to limit the impact of that damage.
• The impact can be physical,
psychological, social (social stigma or
avoidance by others), and financial.
• Strategy at this stage in general is
rehabilitative.
75
76. Levels of Disease Occurrence
Diseases occur in a community
at different levels at a point in time
at predictable levels or in excess of what is expected
1. Expected levels
Endemic: the usual presence of disease from low to
moderate level
Hyperendemic: a persistently high level of disease
Sporadic: Normally does not occur, but occasional cases
occur at irregular intervals
76
77. Cont….
2. Excess of expected levels
Epidemic: An excess occurrence of disease
over the expected level at certain time.
Outbreak: Synonymous with epidemic, but
characterized by a sharp rise and fall in
incidence, limited in area.
Pandemic: An epidemic that affects several
countries or continents.
77
