for health service

1. Epidemiology
Bayisa Abdissa (MPH, Asst. Prof)
June 2023
1

2. Cont…
• Epidemiology is the study of how diseases is
distributed in a population and about factors which
determines or influences this distributions.
• Old definitions of epidemiology: The study of
epidemics or infectious diseases
2

3. Cont…
• Broader definitions:
• The study of the frequency, distribution and
determinants of health and health related
events in specific populations, and the
application of it for the prevention and control
of health problems.
3

4. Cont…
Frequency:
• Example
– Number of exposed people
– Number of people with the outcome of interest
– Proportion of exposed people
– Proportion of people with the outcome of interest
_initial step to investigate risk factors of a diseases
4

5. Cont…
• 2. Distribution
• It is the occurrence of diseases and other
health problems in terms of person, place and
time.
• This attempts to answer Who? Where? And
When?
5

6. Cont…
3. Determinants
• Factors that are related with the occurrence of a
disease in individuals.
• the causative factors for the occurrence of a
disease.
• How diseases occurs among some people and not
among others?
• Premises- diseases and ill health will not
randomly distributed
6

7. Cont..
We all do have characteristics which predispose
or protect us from a diseases
Mainly Genetic or exposure to Evn’t factors
Modifiable and non modifiable:
7

8. Cont…
Host:
Agent: Environment:
 Biological agents  Physical environment
 Biological environment
 Social environment
 Demographic characteristics
 Biological characteristics
 Socioeconomic characteristics
 Infectious diseases result from the interaction
between the infectious agent, host and environment.
8

9. Scope of epidemiology
• Originally epidemiology was concerned with
epidemics of communicable diseases and epidemic
investigations. Later it was extended to study about
endemic communicable diseases and non
communicable diseases.
• Communicable Vs non communicable diseases
9

10. Objectives of Epidemiology
• Identifications of causative agents/risk factors
• To determine burden of diseases in the
community
• Natural history of diseases
• Evaluate Intervention programme
• Policy development
10

11. Cont…
• Epidemiology vs prevention:- a major use of
epidemiologic evidence is to identify a subgroup in a
population at high risk for a diseases?
• Why we identify those at high risk? How??
Early diseases detection and modifying risk factors
Approaches - population based approach vs high risk
based approach
11

12. Cont…
From observation to preventive/policy
1. Ignáz Semmelweis and Childbed Fever
2. Cholera Vs John Snow
Theory of diseases development (Miamsim theory)
3. Smallpox and Edward Jenner
Vaccination and quarantine origin
12

13. 2. Epidemiology of communicable
diseases
What is communicable diseases?
Why communicable?
Why Non communicable
Transitions (Demographic, Epidemiologic and
Nutrition)
13

14. Terminology
 Infection
 Contamination
 Infestation
 Contagious disease
 Epidemic
 Endemic
 Pandemic
 Reservoir
 Host
 vector
 Virulence
 Infectivity
 Pathogenicity
 Incubation period
 Latent period
 Exotic disease
 Sporadic
 Zoonosis
 Nosocomial infection
 Opportunistic infection,
14

15. Cont…
Definition:
 Communicable Disease: is an illness due to a
specific infectious agent or its toxic products
that arises through transmission of that agent or its
products from an infected person or reservoir to a
susceptible host.
15

16. Agent - host relationship
Agent
Reservoir
Mode of
transmission
Host
Habitat where agent survives
or propagates
o Many diseases have
multiple reservoirs and
modes of transmission
Then what is the role of Epidemiology???
16

17. Role of Epidemiology in Communicable Disease
 Identification of causes of infections(agent)
Identification of source or reservoir of disease agent
Studies of routes of transmission and natural history
of infections
 Determine host and environmental factors that
facilitate the development of disease within a defined
population
Identification of best intervention
17

18. 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 process begins with exposure to factors
capable of causing disease
 Without medical intervention, the process ends
with recovery ,disability, or death
18

19. Natural History of Disease cont.
Natural history time lines for infection and disease
Usual time of
screening
19

20. The Spectrum of Illness
In apparent
infection
Mild
Disease
Severe
Disease
Death
No signs or
symptoms
Clinical illness with signs and
symptoms
Recovery
20
Disability
 The course can be halted at any time in the progression
by intervention, host factors and other influences

21. “Iceberg” concept of infectious
disease in populations
EXPOSURE WITHOUT INFECTION
INFECTION WITHOUT
CLINICAL ILLNESS
MILD ILLNESS
SEVERE
DISEASE
DEATH
10
CLINICAL
DISEASE
SUB CLINICAL
DISEASE

22. Chain of Infection
 For infection to occur a chain of events must
take place
Susceptible
host
Causative
agent
Reservoir
Portal of
entry
Portal of
exit
Mode of
transmission
Components of Chain of
Infectious process
1. Causative Agent
2. Reservoir host
3. Portal of exit
4. Mode of transmission
5. Portal of entry
6. Susceptible host
22

23. Chain of Infectious process.
23

24. Chain of Infection ….
Reservoir: The habitat of an infectious agent where
it normally lives, grows and multiplies
 Reservoirs include humans, animals, and the
environment
24

25. Chain of Infection….
 Human reservoirs: Many of the common infectious
diseases have human reservoirs. Diseases which
are transmitted from person to person without
intermediaries include the sexually transmitted
diseases, measles, mumps, streptococcal infection,
most respiratory pathogens, and many others
 Two types of human reservoir exist:
 carriers
 persons with symptomatic illness
25

26. Chain of Infection cont.
• Carrier: A person without apparent disease who is
capable of transmitting the agent to others
 Asymptomatic carrier: transmit infection
without ever showing signs of the disease
 Incubatory carrier: transmit infection by
shedding the agent before the onset of clinical
manifestations
26

27. Chain of Infection cont.
 Convalescent carrier: Transmit infection after the
time of recovery from the disease
 Chronic carrier: Shed the agent for a long period
of time, or even indefinitely
27

28. Time Course of a disease in relation to its Clinical
Expression and Communicability
28

29. Chain of Infection cont.
 Animal reservoirs: Infectious diseases that
are transmissible under normal conditions from
animals to humans are called zoonosis . Such
diseases include brucellosis (cows and pigs),
anthrax (sheep), plague (rodents), trichinosis
(swine), and rabies (bats, dogs and foxes).
29

30. Chain of Infection cont.
 Environmental reservoirs: Plants, soil, and
water in the environment are also reservoirs for
some infectious agents
 For example, soil may harbor agents that causes
tetanus, anthrax and botulism
30

31. Chain of Infection cont.
Portal of exit: is the path by which an agent
leaves the source host
 The portal of exit usually corresponds to the site
at which the agent is localized
 For example, tubercle bacilli and influenza
viruses exit the respiratory tract, schistosomes
through urine, cholera vibrios in feces
31

32. Chain of Infection: Path of Disease
infection
32

33. Chain of Infection cont.
Modes of transmission: refers to the routs of
an infectious agent transfer from an infected host
or reservoir to a susceptible host. -(After an
agent exits its natural reservoir)
 Direct
 Direct contact
 Droplet spread
 Indirect
 Airborne
 Vehicle borne
 Vector borne
33

34. Chain of Infection cont.
 Direct transmission: Immediate direct transfer
of the agent from a reservoir to a susceptible
host by direct contact or droplet spread
Direct transmission
Touching
 Kissing
 Direct projection
 Blood transfusion
 Sexual intercourse
34

35. Chain of Infection cont.
 Indirect transmission: an agent is carried from a
reservoir to a susceptible host by suspended air
particles or by animate (vector) or inanimate (vehicle)
intermediaries
Indirect transmission
 Airborne
 Vector-borne
 Vehicle-borne
Parenteral
injections
35

36. Chain of Infection cont.
Portal of entry: is the path by which an agent
enters a susceptible host
 The portal of entry must provide access to tissues
in which the agent can multiply or a toxin can act
 The portal of entry into the host is usually the
same as the portal of exit from the reservoir
36

37. Chain of Infection cont.
Susceptible host: is the final link in the chain of
infection
People who lack adequate immunity to resist
 Susceptibility of a host depends on genetic
factors, specified acquired immunity, and
other general factors which alter an individual’s
ability to resist infection or to limit Pathogenicity
 An individual’s genetic makeup may either
increase or decrease susceptibility
37

38. Chain of Infection cont.
 General factors which defend against infection
include the intact skin, mucous membranes,
gastric acidity, cilia in the respiratory tract and
nonspecific immune response
 General factors that may increase susceptibility
are malnutrition, alcoholism, and disease or
therapy which impairs the nonspecific immune
response.
38

39. Chain of Infection cont.
 Specific acquired immunity: refers to protective
antibodies that are directed against a specific
agent
 Individuals gain protective antibodies in two ways:
1) They develop antibodies in response to infection,
or vaccine; immunity developed in these ways is
called active immunity.
39

40. Chain of Infection cont.
2) They acquire their mothers’ antibodies before
birth through the placenta or they receive
injections of antitoxins or immune globulin;
immunity that is acquired in these ways is called
passive immunity.
40

41. Chain of Infection cont.
 Note that, the chain of infection may be interrupted
when an agent does not find a susceptible host.
 This may occur if a high proportion of individuals in a
population is resistant (immune) to an agent. These
may happen by reducing the probability of contact
between infected and susceptible persons. This
concept is called herd immunity.
41

42. Chain of Infection cont.
42
 With herd immunity, a large fraction of a population is
immune to a given disease, and it is difficult for the disease
to spread

43. Chain of Infection cont.
 Herd immunity is group immunity that limits the
spread of disease
 The degree of herd immunity necessary to prevent
or abort an outbreak varies by disease
 In theory, herd immunity means that not everyone
in a community needs to be resistant (immune) to
prevent disease spread and occurrence of an
outbreak
43

44. Chain of Infection cont.
 In practice, herd immunity has not prevented
outbreaks of measles and rubella in populations
with immunity levels as high as 85 to 90%
 One problem is that, in highly immunized
populations, the relatively few susceptible persons
are often clustered in population subgroups, usually
defined by socioeconomic or cultural factors. If the
agent is introduced into one of these subgroups, an
outbreak may occur.
44

45. 45
Immune people
Susceptible people
Herd immunity
 Susceptible individuals benefit from an indirect
immunity (not self-made immunity)

46. Chain of Infection cont.
Implications for public health:
 By knowing how an agent exits and enters a host,
and what its modes of transmission are, we can
determine appropriate control measures
 In general, we should direct control measures against
the link in the infection chain that is most susceptible
to interference
 For some diseases, the most appropriate intervention
may be directed at controlling or eliminating the
agent at its source.
46

47. Chain of Infection cont.
Implications for public health
 Sometimes, we direct interventions at the mode
of transmission
 For direct transmission, we may provide
treatment to the source host or educate the
source host to avoid the specific type of contact
associated with transmission
 For vehicle borne transmission, we may
decontaminate or eliminate the vehicle.
47

48. Chain of Infection cont.
For airborne transmission, we may modify
ventilation
For vector borne transmission, we usually
attempt to control (i.e., reduce or eradicate)
the vector population.
48

49. Chain of Infection cont.
 Finally, we may apply measures that protect portals of entry
of a susceptible potential host or reduce the susceptibility of
the potential host
– Mask and gloves are intended to protect the dentist from
a patient’s blood, secretions, and droplets, as well to
protect the patient from the dentist
– Prophylactic antibiotics and vaccination are strategies
to improve a potential host’s defenses.
49

50. Exposure, Infection and Disease Outcome cont.
Outcomes at each stage starting from exposure
Disease
outcome
Exposure Infection Disease
Infectivity Pathogenicity Virulence
 The progress of an infectious agent along the
pathway from exposure to infection, to disease, to
disease outcome can be quantified as follows:
50

51. Exposure, Infection and Disease Outcome cont.
1. From exposure to infection
 Infectiousness: the proportion of an exposed
susceptible host who become infected
(measured by infection rate/infectivity), as:
No. of infected
No. of exposedsusceptible host
X 100
51

52. Exposure, Infection and Disease Outcome cont.
2. From infection to disease
 Pathogenicity: the proportion of infected people
who develop clinical disease, and measured by
the clinical-to sub-clinical ratio, as:
No. of clinical cases
No. of sub-clinical cases
X 100
52

53. Exposure, Infection and Disease Outcome cont.
3. From disease to disease outcome
 Virulence: the proportion of persons with clinical
disease who become severely ill or die, and it is
measured by Case-fatality-rate or hospitalization
rate
Case-fatality-rate =
# of death of a specific disease
Total # of cases of that specific disease
X 100
Hospitalization rate =
# of hospitalized persons of a specific disease
Total # of cases of that specific disease
X 100
53

54. Exercise
 One hundred people attend a wedding feast, and 80 of
them ate apiece of wedding cake. Ten of the participants
were later hospitalized with severe gastroenteritis, but
fortunately none of them died. When the incident was later
investigated , it was found that the wedding cake harbored
the infectious agent responsible for the outbreak.
Immunoglobulin class M antibodies to that agent,
indicative of the recent infection, were found in 60 of the
80 participants who had eaten cake, including the 10
hospitalized ones. Another 20 of the 60 participants with
antibodies had reported experienced diarrhea after having
attended the wedding feast, but not serious enough for
them to report to the health services. From these data,
54

55. Exercise
a) What is the infection rate of the infectious
agent?
a) What is the pathogenicity of the infectious
agent?
a) Calculate the hospitalization rate?
55

56. Patterns of disease occurrence.
 Different diseases, in different communities, show different
patterns of expected occurrence
 Endemic: a persistent level of occurrence with a low to
moderate disease level (the usual presence)
 Sporadic: an irregular pattern of occurrence, with
occasional cases occurring at irregular intervals.
56

57. Cont…
Excess levels
 Occasionally, the level of disease rises above the
expected level
 Epidemic: When the occurrence of a disease
within an area is clearly in excess of the expected
level for a given time period.
57

58. Cont..
• Outbreak: increase in number of cases
– lasts for only a short time, or
– occurs only in a limited area
• Epidemics: is also an excess number of cases
– an epidemic is more general than an outbreak,
– the increase in the number of cases continues far
longer, and
– the cases are distributed across a wider area.
58

59. Overview of Malaria Epidemics in Ethiopia
• A devastating malaria epidemic occurred in 1958,
– three million cases and 150,000 deaths,
• Since 1958, major epidemics of malaria have occurred at
approximately 5-8 year intervals,
• In 1998, a widespread severe malaria epidemic occurred in
most highland as well as lowland areas in the country
– Amhara and SNNP Regional States, 2003
59

60. Causes of Epidemics
• Malaria epidemics can occur as a result of variability or
changes to (infectivity and immunity).
– Generally malaria epidemics occur in places where
there is low and unstable transmission,
– where people have low or no immunity against
malaria
60

61. Cont…
61
• There could be epidemics in high transmission areas
– deterioration of health system, interruption of anti-malarial
measures or migration of non-immune individuals, such as
population movement in search of labor to these areas.
– What else for epidemics??????

62. Cont…
• Unusual local weather phenomena and activities resulting in
environmental modification that increase vector population;
– Increased vulnerability of population from
famine and malnutrition; (controversy from
study)>
– Interruptions of anti-malarial measures which
have kept malaria under control;
– Resistance to anti-malarial medications and/or
insecticide used for vector control.
62

63. Types of epidemics
63
• True epidemics: cyclical epidemics occurring in non immune
area due to climatic changes in arid and semi arid areas.
• Exaggerated seasonal variation: strong seasonal transmission.
Predictable transmission (50% population if no threshold)
• Complex emergences: malaria transmission exacerbated by
population movements ..political instability

64. Forecasting malaria epidemics
64
• Environmental variables like:
• Rainfall,
• satellite based precipitation measures
• Evapotranspiration
• land surface temperature and vegetation indices
are indicative of the presence of conducive environment for the
breeding of vectors and developments of larvae.

65. Epidemics preparedness
65
• Epidemics management committee available/consulted
• Contingency resource: 25% of annual drug requirement should be
kept as contingency
• Recommended supplies (what?)???
• Personnel ??? Trainings
• Outreach and static teams
• Vector control
• Outside help
• monitoring/data collection/reporting

66. Epidemic Detection
• Method 1 is the classic method, based on norm charts and
thresholds.
– This is currently recommended and probably will
continue to be used for
• Method 2 (cluster mapping) will be tested and gradually
introduced,
66

67. Epidemic Burden:
67
• The impact of epidemics
• Why we measure it??
Priority setting for intervention(in the place)
Plan for prevention
Health system performance and research
• How? (event based and time based)
– Estimating the burden of the epidemic (e.g. %
of outpatient visits, number of cases, proportion
of population and area affected).

68. Method 1
• To establish a threshold for ‘normal’ for any given week
– A health facility’s past data by week should be compiled
and
– A threshold determined using the ‘third quartile’
– Current data may then be compared with the threshold.
– If an increase above the weekly threshold is observed, it
implies that there may be an epidemic.
• Under Method 1, an epidemic is defined as: “The occurrence in a health
facility catchment area, clearly in excess of normal expectancy”.
68

69. Epidemics Confirmation
• Initially, the most important information needed for an
assessment will be:
• How many suspected malaria cases (persons) were
documented within a specified time interval (week, month)
within a specific district or kebele
• How many of these suspected malaria cases were tested by
RDT or microscopy?
• How many of the suspected malaria cases tested were also
diagnosed as positive for P.f and other species
69

70. Cont..
70
• How many deaths, hospitalizations and severe malaria cases
occurred?
• Are there adequate supplies of RDTs, AL and chloroquine (and
quinine, rectal artesunate, IV artesunate)?
• If available, compare current malaria case numbers with
previous malaria registry data.

71. Norm Charts and Thresholds to Detect Epidemics
• What is necessary to build the Epidemic Monitoring Chart
– Ready made Epidemic Monitoring Chart or self
prepared chart
– 5 year data of
• The kebele or Health Center catchment area
– If there is no 5 years data
• Use last years data
– If there is no last years data
• use recent week’s collected data
71

72. Threshold
• Why do we need a threshold? It can be very difficult to distinguish
an epidemic from a normal seasonal case increase.
• Once it is apparent that the seasonal case increase is much higher
than normal, the epidemic is well underway.
• How to calculate the threshold? The following tables give
examples of how to tabulate data for estimating a threshold by two
methods.
72

73. Cont…
73
• Does calculating threshold works all the time?
• Situations when it is impossible to use threshold??
• Unstable population vs unavailable previous data
• So what?? –Rapid increase vs health care burden ..failure to handle at
health facility, CFR(%) >1% or 20%
• Confounding to increased number of cases?? -----quality, road, service
charge , other

74. Cont…
• Test 50 clinically suspected patients in a village.
• Make the following decisions:
– Generally, rates exceeding the usual health post and/or
season specific thresholds of RDT or slide positivity
rate should be considered an epidemic.
– In the absence of the above data, if the positivity rate
(RDT or slide) is at least 50% out of at least 50
specimens tested, this is considered as the occurrence
of an epidemic in the health post catchment area and
the team should start urgent mitigation activities.
74

75. Threshold Empty Table
WHO
Week No.
Year 1 Year 2 Year 3 Year 4 Year 5 Third Quartile or second
largest number or 2x last
year’s cases
This year’s cases
1
2
.
.
.
51
52
(53)
Table 18: Chart for assessing usual number of weekly cases (confirmed or clinical) and threshold at health
facility.
75

76. Table 19
WHO
Week No.
Year 1 Year
2
Year
3
Year 4 Year
5
Third Quartile or second largest
number or 2x last year’s cases
This year’s
cases
1 8 42 6 36 14 36 20
2 12 42 27 38 17 38 22
3 10 42 43 49 21 43 35
4 20 17 34 59 32 34 26
5 34 17 46 20 30 34 25
6 18 10 34 22 23 23 20
7 12 19 33 24 25 25 21
8 37 10 27 61 23 37 25
9 32 18 37 29 26 32 16
10 31 24 28 17 13 28 5
11 22 19 22 12 23 22 15
12 17 39 31 22 43 39 25
13 5 19 19 16 21 19 16
14 22 19 28 25 21 25 30
15 29 16 28 19 13 28 45
16 17 32 25 6 11 25 60
17 28 11 32 8 8 28 62
18 17 34 40 13 9 34 60
19 12 17 27 9 10 17 25
20 16 18 14 1 9 16 10
21 31 34 29 2 8 31 15
22 38 22 23 1 9 23 16
23 29 33 14 1 17 29 17
24 19 32 35 1 32 32 18
25 27 10 25 1 34 27 22
26 36 20 34 1 47 36 30
76

77. Table 20: Doubling recent year data
WHO
Week No.
Year 1 Year 2 Year 3 Year 4 Year 5 Threshold
(norm) = 2x last
year’s cases
This Year’s
cases
1 14 28 20
2 17 34 22
3 21 42 35
4 32 64 26
5 30 60 25
6 23 46 20
7 25 50 21
8 23 46 25
9 26 52 16
10 13 26 5
11 23 46 15
12 43 86 25
13 21 42 16
14 21 42 30
15 13 26 45
16 11 22 60
17 8 16 62
18 9 18 60
19 10 20 25
20 9 18 10
21 8 16 15
22 9 18 16
23 17 34 17
24 32 64 18
25 34 68 22
26 47 94 30
77

78. 78

79. Figure using 5 years
Number
of
malaria
cases
Week number
Threshold third quartile This year
79

80. Figure Using recent year
Number
of
malaria
cases
Week number
Threshold 2x last year This year
80

81. Response to malaria outbreaks
81
• The basic strategies are containment and mitigation.
• Containment: during the early stages of an outbreak including
case tracing and isolation to avoid its spread.
• Vaccination
• Mitigation:- when it became evident that it is no longer
possible to contain the epidemic.

82. Cont…
82
• The major aim is to flatten an epidemic curve(health system
burden)
• Include non pharmacological measures like social mobilization
and behavioral changes to use the

83. Exercise
Week
no.
(EFY)
Week no
(WHO)
2003 2004 2005 2006 2007 2008
second largest
number
This year (2009)
1 28 16 42 105 36 14 42 33
2 29 12 42 100 38 17 22 35
3 30 16 42 103 49 21 34 40
4 31 20 17 134 59 32 40 39
5 32 34 17 146 20 30 39 33
6 33 18 10 134 29 23 27 30
7 34 30 19 133 24 25 25 29
8 35 37 10 127 41 23 42 42
9 36 32 18 137 29 26 29 35
10 37 31 24 128 17 13 32 30
. . . . . . . .
51 26 26 40 134 32 39 39 .
52 27 23 35 110 27 25 33 .
Weekly malaria cases in 2004–2008 (EFY).
83

84. Exercise
• Which year do you think the data shows an abnormally high number of
malaria cases? What do you do with this year before you start identifying
the second largest number?
• Identify the second largest number for the six years of data (2003 –2008)
and fill in the column in the table.
• Use the blank epidemic monitoring chart and plot a reference line of the
second largest numbers and the data for the year 2009 against it.
• Does the graph show weeks when an epidemic occurred? If yes, in which
weeks?
84

85. 0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51
Numberof
malaria
cases
EFY week number Start @ 1 & WHO week number start @ 28
Malaria Epidemic Monitoring Chart
second largest number
This year
85

86. 0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51
number
of
malaria
cases
EFY Week number start @ 1 or WHO Week numbers start @ 28
Malaria Epidemic Monitoring Chart
Threshold 2 X last year
This year
86

87. Disease prevention
 It involves the interruption or slowing of disease
progression through appropriate intervention
 There are several stages during the course of a
disease at which we can intervene in order to
control the disease
 The concept of prevention is best defined in the
context of levels
87

88. Disease prevention cont.
Level of disease prevention
 Three levels depending on when in the
course of disease interventions are made:
1. Primary prevention
2. Secondary prevention
3. Tertiary prevention
88

89. Disease prevention cont.
1. Primary prevention
 Action taken prior to the biological onset of a
disease, which removes the possibility that the
disease will ever occur
 The objectives here are to promote health,
prevent exposure, and prevent occurrence of
disease
 Promotive and preventive.
89

90. Disease prevention cont.
1.1. Health promotion: Intervention that enhance health
and the body’s ability to resist disease. In short, any
intervention which promotes a healthier and happier life.
– Health education
1.2. Prevention of exposure:
– Provision of safe water for water borne diseases
– Proper excreta disposal for faeco-oral diseases
– Provision of vector control for vector borne diseases ;
 Use of bed nets to control anopheles mosquito
– Consistent use of condom for STI
90

91. Disease prevention cont.
1.3. Prevention of disease occurrence:
 Interventions that act during the latency period
between exposure and the biological onset of
disease
 Intervention which acts at this stage does not
prevent an infectious agent from invading its
human host, but does prevent it from establishing
an infection.
– Immunization for vaccine preventable diseases
91

92. Disease prevention cont.
2. Secondary prevention:
 Interventions that act after the biological onset
of a disease, but before permanent damage
sets in
 The objective here is to stop or slow the
progression of disease so as to prevent or limit
permanent damage.
E.g. Early detection and treatment of the
disease
 Can prevent clinical onset or permanent damage,
depending on the disease and type of intervention
92

93. Disease prevention cont.
3.Tertiary prevention:
 Intervention that acts after permanent damage has
set in, and the objective here is to limit the impact
of that damage
 The impact can be physical, psychological,
social(social stigma) and financial.
 Tertiary prevention can prevent deterioration in
quality of life and or early death
93

94. 94

95. 95