1. CONCEPT OF DISEASE
• A condition of the body or some part
or organ of the body in which its
functions are deranged.
• It is a mal-adjustment of human
organism to the environment.
• I t is deviation from normal function.

3. CONCEPT OF CAUSATION
• DEMONISTIC THEORY
• DEVILISITIC THEORY
• TRIDOSHAS THEORY
• FOUR HUMOURS THEORY
• YANG and YIN Principles
• GERM THEORY - Louis Pasteur, Robert Koch
• EPIDEMIOLOGICAL TRIAD
• MULTI FACTORIAL THORY – Web of disease
causation, Wheel of causation

4. Henle-Koch’s Postulates
1. The agent should be present in every
case of the disease under appropriate
condition
2. The agent should not be present in any
other disease as a fortuitous and Non-
Pathogenic agent
3. The agent must be isolated from the
body of the individual in pure culture
4. It should induce disease in a new
susceptible experiment animal

6. NATURAL HISTORY OF DISEASE
• IT IS 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

7. NATURAL HISTORY OF DISEASE
• PRE-PATHOGENIC PHASE OR
SUSCEPTIBILITY STAGE
• PATHOGENIC PHASE
1. INCUPATION PERIOD
2. PRODROMAL STAGE
3. STAGE OF OVERT DISEASE
4. STAGE OF DEFERVESCENCE
5. STAGE OF CONVALESCENCE

8. FACTORS AFFECTING THE
GRADIENT OF INFECTION
• Infectivity
• Pathogenicity
• Virulence
• Antigenicity

9. TYPE OF INFECTION
• Latent infection
• Sub-clinical infection or
inapparent or occult
• Atypical infection
• Severe clinical infection

10. Factors for development or spread
of infectious disease
• An etiological agent responsible for the disease
should be present
• There should be a reservoir or carrier for the
etiological agent to survive
• The infecting agent should be able to escape
from the reservoir of infection through the portal
of exit
• There should be a possible source of entry to
transmit the agent to a new susceptible host
• The agent should be able to invade the new host
• The host should be susceptible

11. Background
• Infectious disease epidemiology
– the occurrence of infectious disease in a given host is
dependent on the presence of disease in other members of the
population and the length of time that infected hosts are able to
transmit disease to others
– understanding these characteristics of a
disease allow us to develop rational measures to control disease

12. Definition & Stages
• Definition ; The course of a disease from onset
(inception) to resolution.
• Stages
Progress to a fatal termination
Stage of
Pre-symptomatic Clinically
pathologic Remission and relapses
stage manifest disease
onset
Regress spontaneously,
leading to recovery
Risk Factors Precursors Effect of Treatment Prognostic factor

13. Risk factor
• Risk factor; An aspect of personal behavior or life style,
an environmental exposure, or an inborn or inherited
characteristic, that, in the basis of epidemiologic
evidence, is known to be associated with health-related
condition (s) considered important to prevent.
– Risk marker; increased probability of a specified
outcome; not necessarily a causal factor
– Determinant; can be modified by intervention,
thereby reducing the probability of occurrence of
disease or other specified outcomes

14. The Natural history of disease in a patient
Preclinical Phase Clinical Phase
(A) (P) (S) (M) (D) (T)
• A ; Biologic onset of disease
• P ; Pathologic evidence of disease if Sought
• S ; Signs and symptoms of disease
• M ; Medical care sought
• D ; Diagnosis
• T ; Treatment
Gordis L. Epidemiology. WB Saunders Company. 1996

15. THE NATURAL HISTORY OF A DISEASE
STIMULUS to
HOST REACTION RECOVERY
the HOST
interrelation of
Agent, Host and Latent Period (Pre- Symptoms, with or without Defects,
Environmental symptomatic) Signs(Clinical) Disability
factors
PREPATHOGE
PERIOD OF PATHOGENESIS
NESIS
Health
Promotion Disability Limitation
Specific Early Diagnosis and Prompt
Protection Treatment,
Rehabilitation
PRIMARY SECONDARY
TREATMENT TERTIARY PREVENTION
PREVENTION PREVENTION
(Leavell's Level of Application of Preventive Medicine)

16. TIME
Death
Infection Clinical disease
Susceptible
host Recovery
No infection
Incubation period
Latent Infectious Non-infectious
Exposure Onset

17. • Latent period
the time interval from infection to development of
infectiousness
• Infectious period
the time during which time the host can infect another
susceptible host
• Non-infectious period
the period when the host’s ability to transmit disease to
other hosts ceases
• Incubation period
the time interval between infection to development of
clinical disease

18. • e.g : Chicken pox
– an infectious disease caused by the varicella-
zoster virus
– the latent period for chicken pox is shorter than
the incubation period, so a child with chicken pox
becomes infectious to others before developing
symptoms

19. TIME
Death
Infection Clinical disease
Susceptible
host Recovery
No infection
Incubation period
Latent Infectious Non-infectious
Exposure Onset

20. • Other examples?
– HIV (AIDS)
• latent period relatively short
• infectious period occurs (many years) before the onset
of symptoms

21. TIME
Death
Infection Clinical disease
Susceptible
host Recovery
No infection
Incubation period
Latent Infectious
Exposure Onset

22. e.g : Malaria
– caused by protozoan parasites of the genus
Plasmodium
– the stages of the parasite that are infective to
mosquitoes occur about 10 days after the
development of symptoms
– latent period is around 10 days longer than the
incubation period, so early treatment of
symptoms could have an important effect on
transmission

23. Natural history of disease
TIME
Death
Infection Clinical disease
Susceptible
host Recovery
No infection
Incubation period
Latent Infectious
Exposure Onset

24. Latent Period of Chronic
Disease
• Definition; "Interval between exposure to a disease-
causing agent and the appearance of manifestations
of the disease"
• cf. incubation period in infectious disease
1) brief exposure
Two conditions
2) prolonged or continuous exposure

25. Primary Prevention
• 'Preventing the occurrence of disease or injury by
modifying risk factors.'
• 'Various aspects are considered to produce
effective primary prevention program. Especially,
advancing knowledge of disease causation must
be required.‘

26. Primary Prevention
• ** Guidelines for effective prevention programs(RB Wallace,
GD Everett,1986)
– Programs must be based on scientific evidence.
– Prevention programs should be supported by effective data
system.
– Programs should be flexible.
– Programs must be sensitive to ethical issues.
– Programs should be targeted to the recipients most in need.
– Programs should muster a variety of community resources.
– Effective prevention requires legislative action and social policy
decisions.
– Programs should be continuous.

27. Primary Prevention
• General health promotion
– 'Proper nutrition, mental hygiene, adequate housing, and appropriate
balance between work and play, est and exercise, and useful and
productive place in society, are among the best recognized factors
ontributing to maintenance of optimum health.(Commission on Chronic
illness, USA, 1957)‘
• Specific protection
• Health Promotion
– 'Health promotion is any combination of educational, organizational,
economic, and environmental supports for behavior and conditions of
living conducive to health (LW Green, 1992).'

28. Criteria for the Development of Health
Promotion and Education Programs
• A health promotion program should address one or more risk
factors which are carefully defined, measurable, modifiable,
and prevalent among the members of a chosen group, factors
which constitute a threat to the health status and the quality
of life of target group members.
• A health promotion program should reflect a consideration of
the special characteristics, needs, and preferences of its
target groups(s)
From APHA Technical Report

29. Criteria for the Development of Health
Promotion and Education Programs
• health promotion programs should include interventions which will clearly
and effectively reduce a targeted risk factor and are appropriate for a
particular setting
• A health promotion program should identify and implement interventions
which make optimum use of available resources.
• From the outset, a health promotion program should be organized,
planned, and implemented in such a way that its operation and effects can
be evaluated.

30. MODES OF TRANSMISSION
• DIRECT • INDIRECT
TRANSMISSION TRANSMISSION
• DIRECT CONTACT • VECHICLE BORNE
• VECTOR BORNE
• DROPLET INFECTION
• A) mechanical
• CONTACT WITH SOIL
• B) biological
• INOCULATION INTO
• AIR-BORNE
SKINOR MUCOSA
• FOMITE BORNE
• TRANSPLACENTAL
• UNCLEAN HANDS AND
FINGERS

31. BIOLOGICAL TRANSMISSION
• Propagative
• Cyclo-Propagative
• Cyclo-developmental
• Transovarial transmission
• Trans-stadial transmission

32. SOURCE OF INFECTION
• It is defined as the person,
animal, object or substance
from which an infectious
agent passes or is
disseminated to the host

33. RESERVOIR
• It is defined as “any person, animal,
arthropod, plant, soil, or substance
“(or combination of these in which an
infectious agent lives and multiplies,
on which it depends primarily for
survival, and where it reproduces itself
in such manner that it can be
transmitted to a susceptible host”

34. CARRIERS
A Carrier is defined as an infected
person or animal that harbours a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential
source of infection for others

35. CARRIERS
• TYPE • PORTAL OF EXIT
A) Incubatory A) Urinary
B) Convalescent B) Intestinal
C) Healthy C) Respiratory
• DURATION
A)Temporary
B)Chronic

36. FEATURES OF CARRIER
1. Presence of specific microbes in
the body
2. Absence of apparent symptoms
and signs
3. Shedding of micro-organisms in
the discharges or excretions
4. As a source of infection to others

37. INCUPATION PERIOD
THE TIME INTERVAL BETWEEN
INVASION BY AN INFECTIOUS
AGENT AND APPEARANCE OF
THE FIRST SIGN OR SYMPTOM
OF THE DISEASE IN QUESTION

38. FACTORS AFFECTING THE
INCUPATION PERIOD
• DOSE OF INOCULUM
• SITE OF MULTIFICATION
• RATE OF MULTIFICATION
• HOST DEFENCE MECHANISM

39. FACTORS TO DETERMINE THE
INCUBATION PERIOD
• GENERATION TIME
• INFECTIVE DOSE
• PORTAL OF ENTRY
• INDIVIDUAL SUCEPTIBILITY

40. IMPORTANCE OF INCUPATION
PERIOD
• Tracing the source of infection and
contact
• Period of surveillance
• Immunization
• Identification of point source or
propagated epidemics
• Prognosis

41. MEDIAN INCUPATION PERIOD
• IT IS DEFINED AS THE TIME
REQUIRED FOR 50% OF THE
CASES TO OCCUR
FOLLOWING EXPOSURE

42. LATENT PERIOD
IT HAS BEEN DEFINED
AS THE PERIOD FROM
DISEASE INITIATION TO
DISEASE DETECTION

43. GENERATION TIME
IT IS DEFINED AS THE INTERVAL OF
TIME BETWEEN RECEIPT OF
INFECTION BY A HOST AND MAXIMAL
INFECTIVITY OF THAT HOST

44. SERIAL INTERVAL
THE GAP IN TIME
BETWEEN THE ONSET OF
THE PRIMARY CASE AND
THE SECONDARY CASE

45. COMMUNICABLE PERIOD
It is defined as the time during
which an infectious agent may be
transferred directly or indirectly
from an infected person to another
person, from an infected animal to
man , or from an infected person to
an animal, including arthropods

46. SECONDARY ATTACK RATE
It is defined as the number of
exposed persons developing the
disease within the range of the
incubation period, following
exposure to the primary case

47. Number of exposed persons developing the SAR
= disease within the range of the incubation period  100
Total number of exposed /susceptible

48. HERD IMMUNITY
• IT IS THE LEVEL OF RESISTENCE
OF A COMMUNITY OR GROUP OF
PEOPLE TO A PARTICULAR
DISEASE

49. BEHAVIOUR OF DISEASE IN THE
COMMUNITY
1. EXOTIC
2. SPORADIC
3. ENDEMIC
4. EPIDEMIC
5. PANDEMIC
6. OUTBREAK

50. EPIDEMIC
It is the unusual occurrence in a
community or region of cases of an
illness, specific health-related
behavior, or other health related
events clearly in excess of normal
expectancy (LAST,1995)

51. OUT BREAK
TWO OR MORE RELATED
CASES IN INFECTIONS,
SUGGESTING THE
POSSIBILITY OF A COMMON
SOURCE OR TRANSMISSION
BETWEEN CASES

52. FACTORS IN DISEASE CAUSATION
1. Predisposing factors age, sex and previous
illness
2. Enabling factors low income,
poor nutrition, bad housing, inadequate medical care
3. Precipitating factors exposure to a specific
disease agent
or noxious agent
4. Reinforcing factors repeated exposure, unduly hard
work

53. THE EPIDEMIOLOGIC TRIANGLE
"triad" that play a role in disease process
Agent
Vector
Host Environment
TRADITIONAL MODEL OF INFECTIOUS DISEASE CAUSATION

54. AGENT
As an element or substance, animate or inanimate,
the presence (or absence) of it may initiate or perpetuate a disease process

55. HOST
A person or other living animal, that affords subsistence or lodgment to an
infectious agent
under natural condition
Host factors
Intrinsic factors that influence an individual’s exposure, susceptibility, or
response to a causative agent

56. ENVIRONMENT
As the aggregate of all the external conditions
and influence affecting the life and development of an organism
Environmental factor
Extrinsic factors which affect the agent and the opportunity for
exposure

57. AGENT
a. Nutritional agent
carbohydrate, vitamin, fat, protein, mineral, water
Example :
- diabetes mellitus, obesitas, hyperlipidemia, kwashiorkor
- avitaminosis
- cretinism, anemia
- edema, dehydration

58. b. Chemical agent
polutan , drugs, Hg, Pb, Ag, arsenicum,
pesticide (Chlorinated Hydrocarbon CCl4 : DDT, endrin, dieldrin and organo
hosphate, diacynon, malathion, butazinon), cosmetics, etc.
c. Physical agent
collision, traffic accident, falling down, dust,
climate (frost bite, heat stroke)

59. d. Infectious agent
- Virus : dengue, morbili, varicella, hepatitis
- Ricketsia : typhus exanthematicus, Rocky Mountain
Spotted Fever, scrub typhus (rat-bite fever)
- Bacteria : gram (+), gram (-) ; bacil, coccus,
acid fast resistence, anaerob, etc.
- Fungi : tinea capitis, tinea cruris, tinea pedis
- Protozoa : plasmodium, amoeba
- Metazoa : worm (ascaris , ancylostoma, etc.)

60. HOST
Intrinsic factors that play a role in disease process
- age
- sex
- religion
- customs
- occupation
- marital status
- family background
- genetic-hereditary
- ethnic / race
- physiologic / psychological status
- habit / behavior
- immune status
- previous disease

61. ENVIRONMENT
1. Physical environment
geographic, geology, climate
2. Biological environment
people, flora, fauna, food population density
3. Socioeconomic
income, education, culture, urbanization, economic
growth, poverty, fertility, etc.

62. NATURAL HISTORY AND SPECTRUM OF
DISEASE

63. Natural history of disease
The progress of a disease process in an individual overtime in the absence of
intervention
recovery
Exposure host disease disability
death

65. INCUBATION PERIOD
The time interval between contact with an agent and the first clinical evidence
of resulting disease
Depends on :
• Portal of entry (defense mechanism)
• The ability of multiplication (infectivity)
• Number of agents
• Level of antibody in the host
It varies individually

66. Type of incubation period in disease outbreak
number of
cases
A B
time
A : skewed to the left the disease has a
short incubation period
B : skewed to the right the
disease has a longer incubation period

67. DEFENCE MECHANISM
THE ABILITY TO REACT AGAINST AGENT INVASION IN THE BODY
Consist of :
• The external defense mechanism : physical and
chemical reaction
• The internal defense mechanism :
cellular and humoral immunity

68. EXTERNAL BARRIER
Respiratory tract sense of smell, cough and
sneeze reflex, mucous membrane, hair of the nose, ciliated epithelium.
Small particles < 5 can enter directly into the alveoli.
Digestive tract sense of taste, vomit reflex, gastric
acid fluid, peristaltic of intestine and diarrhea
Skin
structure of the skin, sebaceous glands, apocrine and accrine sweat glands,
hair
Eye
blink reflex, eye brow, eye lash, tears

69. If the external barrier can not eliminate the agent
internal defense mechanism will continue the defence mechanism process by
:
- Inflammation
- Isolation by fibrocyte
- Macrophage phagocytosis
- Antibody reaction
Immunization a way to increase the internal
defence mechanism

70. The natural history and spectrum of disease challenges to the clinician
and to the public health worker
To the clinician
Because of cases diagnosed by clinicians in the community often represent only the “tip
of the iceberg”, it is important to do the “case finding” and report it to the public
health worker
To the public health worker
While searching the rest of the cases, they should prevent disease transmission and
outbreak

71. ICEBERG PHENOMENE
CLINIC
CLINICAL
HORIZON
SUB CLINIC CURE
The proportion of sub clinical patients are greater in number than the
patients with complete symptoms
This portion should be early detected, because it has the capability of
transmitting the disease causing outbreak

72. CELL RESPONSE HOST RESPONSE
Lysis of cell Death of organism
Clinical disease
Inclusion body formation
Discernable effect
Classical and severe disease
or
cell transformation
or
Moderate severity
cell dysfunction
mild illness
Viral multiplication Infection without
without visible clinical illness
change or (asymptomatic infection)
Subclinical disease
incomplete viral
Below visual change
maturation
Exposure without Exposure without
attachment and/or infection
cell entry

73. CHAIN OF INFECTION

74. AGENT CHARACTERISTIC
1. Natural characteristics of the agent
The morphology, physiology, reproduction, motility, metabolism, need of oxygen ,
temperature, production of toxin, antigen, living cycle, reaction against
physical and chemical substance
2. Characteristic of the agent related to infection in human
a. infectivity
b. pathogenicity
c. virulence
d. antigenicity
e. tropism

75. 3. Reservoir of agent
4. Portal of entry and portal of exit
5. The incubation period
6. The spread of the disease
7. Natural cycle of infection

76. INFECTIVITY
The ability of agent to attack, adapt, live and multiplicate in the host
PATHOGENICITY
The ability of agent to produce a local or general reaction in the host
VIRULENCE
The ability to elicit a severe clinical manifestation

77. ANTIGENICITY
The agent’s ability to stimulate host production of antibody such as agglutinin,
opsonin, precipitin, antitoxin, lysine, complement fixating substance,etc.
Disease with high antigenicity can be prevented by immunization
Example :
• Typhoid fever, morbili : highly antigenic
• Tuberculosis : doubtful
• Influenza virus has lots of strain :
rather difficult to develop an effective vaccine

78. Agent with high infectivity and pathogenicity but low antigenicity will cause a
relatively high disease prevalence in the community
Agent with high infectivity but low pathogenicity usually produce a mild or sub clinical
symptom and carrier
TROPISM
The agent preference to attack and stay in special location in the host
• Cholera : digestive tract
• Staphylococcus : mostly in the skin
• Herpes zoster : nerve system
• Poliomyelitis : anterior-horn cells of spinal cord

79. HERD IMMUNITY
The immunity of a group of people / community.
The resistance of a group to invasion and spreading of an infectious agent
based on the resistance to infection of a high proportion of individual members of the
group.
The herd immunity reduces the susceptibility to infection or can resist a communicable
disease epidemic.
The higher herd immunity the higher the power to defence of an epidemic
occurrence.

80. The high incidence of communicable disease can be due to :
the high proportion of the susceptible individual
or the low portion of herd immunity in
the population
The practical aspect of the concept of herd immunity : the necessity of
immunization program for the whole population to prevent the occurrence of an
epidemic

81. RESERVOIR
Habitat in which an infectious agent normally lives, grows and multiplies
1. HUMAN RESERVOIR
2. ANIMAL RESERVOIR
3. ENVIRONMENTAL RESERVOIR

82. 1. Human reservoir
- Persons with symptomatic illness
- Carrier
Carrier : a person without apparent disease who is nonetheless capable of transmitting
the agent to others
a. Asymptomatic carrier (never show symptoms during the
time they are infected)
b. Incubatory / convalescent carrier (who are capable of transmission
before or after they are clinically ill)
c. Chronic carrier (who continues to harbor an
agent)

83. 2. Animal reservoir
Infectious disease that are transmissible under normal conditions from animals to human
are called zoonoses
• Dog, cat, ape : rabies
• Rat : rat bite fever, plaque, leptospirosis
• Cattle : sheep, goat, camel, cow, pig
(anthrax, brucellosis, bovine tuberculosis, tularemia,
ring worm)
• Arthropode : flies, cockroach, mosquito

84. 3. Environmental reservoir (soil)
• Clostridia (tetanus, botulism, welchii)
• Fungi
• Bacteria
(dust particle : mycobacterium tuberculosis)
• Parasite (helminthiasis)

85. PORTAL OF ENTRY AND PORTAL OF EXIT
The path by which an agent enters of leaves the source host.
Usually corresponds to the site at which the agent is localized.
It is necessary to understand about it because it related to how the disease being
transmitted in other way we can assume how the prevention of the disease.

86. Portal of entry
- digestive tract
- respiratory tract
- skin
- genital
- eye
- blood vessel system
The portal exit seem to be the same with the portal entry, sometimes some disease
have other way of exit beside the former way.
Hepatitis infectiosa, typhus abdominalis :
beside come out by fecal also can be detected in urine and blood.

87. TRANSMISSION
4 transmission ways :
1. Contact transmission
a. Direct transmission :
by mucous contact
e.g. genital-genital, oral-genital, oral-oral
b. Indirect :
hand-mouth, droplet transmission

88. 2. Vehicle transmission
- Transmission by common vehicle :
food, fluid, milk, blood, serum, vaccine
- The agent can be transmitted by ingestion, injection or
inoculation
3. Vector transmission
The arthropods have a role in this transmission
4. Air borne
- droplet nuclei
- dust

89. Transmission of Dengue Virus
by Aedes aegypti
Mosquito feeds / Mosquito refeeds /
acquires virus transmits virus
Extrinsic Intrinsic
incubation incubation
period period
Viremia Viremia
0 5 8 12 16 20 24 28
DAYS
Illness Illness
Human #1 Human #2

90. Replication and transmission
of Dengue virus (part 1)
1. Virus transmitted 1
to human in mosquito
saliva
2
2. Virus replicates
in target organs
4
3. Virus infects white 3
blood cells and
lymphatic tissues
4. Virus released and
circulates in blood

91. Replication and transmission
of Dengue virus (part 2)
5. Second mosquito 6
ingests virus with blood
6. Virus replicates
in mosquito midgut
and other organs, 7
infects salivary
glands
5
7. Virus replicates
in salivary glands

92. Theoretically, the spreading of the disease can be stopped by cutting off every step
of the disease phase.
The principle of communicable disease control is to cut off the chain of transmission
of the disease.
Example :
1. To cut the connection between the reservoir and
the host (contact person) by :
• individual hygiene
• environmental sanitation
2. To increase the defense mechanism by :
• immunization
• nutrition
3. In case the colonization has been occurred :
• early diagnosis & prompt treatment
• screening : malaria, STD, HIV-AIDS

93. IMPLICATION
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, including
prevention methods
FOR CLINICAL/HOSPITAL SETTING
Patients may be treated and/or isolated with appropriate
“precautions”

94. NATURAL HISTORY OF ANY DISEASE
PREPATHOGENIC PATHOGENIC
D E A T H
CLINICAL 1 2 3
Agent Host
Environment Clinical Horizon
interaction
Sub Clinic Convalescence Cured + Sequel
PRIMARY SECONDARYPREV TERTIARY
PREVENTION I ENTION II PREVENTION III

95. STAGE OF PREVENTION
I. PRIMARY PREVENTION
“Health promotion and specific protection”
A. Health Promotion
1. Health education
2. Nutrition
3. Development
4. Housing
5. Marriage counseling
6. Genetic
7. Periodic physical examination

96. B. Specific protection
1. Immunization
2. Personal hygiene
3. Environmental sanitation
4. Occupational hazard
5. Protection to accident
6. Specific nutrition
7. Protection to carcinogen
8. Avoidance of allergic material

97. II. SECONDARY PREVENTION
“Early diagnosis and prompt treatment”
1. Case finding
2. Screening survey
3. Selective examination
a. Cure and prevent
b. Preventing the spread
c. Preventing complication and sequel
d. Shorten of disability

98. Natural history of disease
Onset of Usual time of
symptoms diagnosis
Exposure
Pathologic
changes
Stage of Stage of Stage of Stage of
susceptibility subclinical clinical recovery,
disease disease disability or
death
PRIMARY
PREVENTION SECONDARY
PREVENTION TERTIARY
PREVENTION

99. The natural history of disease
STAGE 1: Susceptibility
DESCRIPTION: Risk factors which assist
the development of
disease exist, but disease
has not developed
EXAMPLE: Smoking

100. The natural history of disease
(cont’d)
STAGE 2: Presymptomatic disease
DESCRIPTION: Changes have occurred
to lead toward illness but
disease is not yet
clinically detectable
EXAMPLE: Alveoli deteriorate

101. The natural history of disease
(cont’d)
STAGE 3: Clinical Disease
DESCRIPTION: Detectable signs and/or
symptoms of disease exist
EXAMPLE: Emphysema detected by
pulmonary function test

102. The natural history of disease
(cont’d)
STAGE 4: Disability
DESCRIPTION: Disease has progressed to
the point of causing a
residual effect
EXAMPLE: Person has difficulty
breathing

103. LEVELS OF PREVENTION
LEVEL: Primary
DESCRIPTION: Promote general health
and avoid risk factors for
disease --- Utilize protective
measures to prevent
susceptibility and
presymptomatic disease
EXAMPLE: Stop smoking or choose
not to start; avoid areas
where people are smoking

104. LEVELS OF PREVENTION
(cont’d)
LEVEL: Secondary
DESCRIPTION: Early detection and
timely treatment
EXAMPLE: Routine pulmonary
function tests for those at
risk; medicine to help
patients breath more
easily; smoking cessation
programs if patient
smokes

105. LEVELS OF PREVENTION
(cont’d)
LEVEL: Tertiary
DESCRIPTION: Rehabilitation and
prevention of further
disease or disability
EXAMPLE: Oxygen therapy;
facilitating ambulation
with technical devices

106. PREVENTION APPROACHES
Population-Based Approach:
• Preventive measure widely applied to
an entire population (public health
approach)
• Strive for small absolute change
among many persons
• Must be relatively inexpensive and
non-invasive

107. PREVENTION APPROACHES
High-Risk Approach:
• Target group of individual at high risk
• Strive for strong risk factor control
• Often times requires clinical action to
identify the high risk group and to
motivate risk factor control.

108. LEVELS OF PREVENTION (Review)
PRIMARY PREVENTION
Prevention of disease by
controlling risk factors (e.g.,
non-smoking promotion)

109. LEVELS OF PREVENTION (Review)
SECONDARY PREVENTION
Reduction in consequences of disease
by early diagnosis and treatment
(e.g., cervical cancer screening)