The document discusses the natural history and transmission of infectious diseases through an epidemiological framework. It describes the infectious disease cycle involving an infectious agent, susceptible host, and conducive environment. There are typically four stages in the natural history of a disease: susceptibility, pre-symptomatic infection, clinical symptoms, and resolution or disability/death. Understanding the disease transmission and progression is important for effective prevention and control strategies.

1. CHAPTER TWO
EPIDEMIOLOGY OF COMMUNICABLE DISEASES

2. Learning objectives
By the end of this chapter, students will be expected
to:
• List the major components of the infectious
disease cycle
• Describe natural history and time course of an
infectious disease
• Describe the type of carriers and roles in the
infectious disease transmission
Describe the different modes of disease
transmission
Describe the different methods of controlling
communicable diseases

3. Infectious diseases
• Infectious diseases: are caused by pathogens
that are transmitted either directly between
persons or indirectly via a vector or the
environment
• An infectious disease is defined as a disease
caused by an infectious agent or its toxic
products

4. Infectious Disease Cycle
• The spread of an infectious disease through
populations is determined by characteristics of the
infectious agent, the host, and the environment-
Epidemiological Triangle
• It refers to the process by which infectious
diseases are transmitted from infectious host to
susceptible host
• It is also called transmission cycle or chain of
infection

7. Disease causation and models:
• Cause of disease: is an event, condition,
characteristic or a combination of these factors
which plays an important role in producing the
disease.
Characteristics of a cause
1. Must precede the effect
2. Can be either host or environmental factors
e.g., conditions, actions of individuals, events,
natural, social or economic phenomena
3. Positive (presence of a causative exposure –
smoking for lung ca) or negative (lack of a preventive
exposure- immunization for TB)

8. Principles of Causation
There are two principles of disease causation.
1. The single germ theory - a disease is caused
by single micro organism
• Luis Pasteur isolated microorganism. This
discovery led to Koch's postulate in 1877.
• Koch’s postulate is a rule for the
determination of causation

9. Koch's Postulate states that:
1.The organism must be present in every case.
2.The organism must be isolated and grown in culture
3.The organism must[when inoculated into a
susceptible animal] cause the specific disease.
4.The organism must then be recovered from the
animal.
Koch’s postulate embodies the idea of specificity of a
cause.
That is, a one to one relationship between an
exposure and a disease.

10. 2. The ecological approach – “Multiple causation"
• For infectious diseases to occur there are three
essential factors,
1.The etiologic agent;
2. Suitable environment for spread and growth of
the agent
3. Susceptible host to invade, multiply and produce
disease

11. • In the ecological view, an agent is considered
to be necessary but not sufficient cause of
disease
-because the conditions of the host and
environment must also be optimal for a disease
to develop.

12. The causes of disease can be classified in to two:
1. Primary causes –are the factors which are
necessary for a disease to occur [if absent the
disease will not occur].
• The term ”etiologic agent” can be used
instead of primary cause for infectious causes
of diseases.
• E.g. “M. tuberculosis” is the primary cause
(etiologic agent) of pulmonary TB.

13. Necessary Vs sufficient cause
a. Sufficient cause:
- a set of minimal conditions and events that inevitably
produce or initiate an outcome or disease
b. Necessary cause – an outcome or disease can not
develop in its absence
E.g. -Tubercle bacilli is a necessary factor for
tuberculosis.
-Rabies virus is sufficient for developing clinical rabies

14. Etiology of disease: All factors that contribute to
the occurrence of a disease.
• They are related to the agent, host and
environment.

15. 2. Risk factors (contributing, predisposing, or
aggravating factors)
• These are not the necessary causes of disease
but they are important for a disease to
occur/may create a state of susceptibility to the
disease agent.
E.g. age, sex and previous illness
• Risk factors could be related to the agent, the
host and the environment.

16. • The etiology of a disease is the sum total of all
the factors (primary causes and risk factors)
which contribute to the occurrence of the
disease.
• Agent factors +Host factors +Environmental
factors = Etiology of a disease
• It is the interaction of the agent, the host, and
the environment which determines whether a
disease develops or not

17. Conceptual Models of Causation
• Depict multi-factorial causation, interdependence
of effects, direct and indirect effects, levels of
causation, and systems or webs of causation
• Models – the epidemiologic triangle, web of
causation, wheel model

18. The Epidemiologic Triangle
• Was widely used for many years and still referred
frequently in epidemiological literature
• Consists of three components: host,
environment and agent
• Each component must be analyzed and
understood for prediction of patterns of a disease
• This model highlights the agent of disease as a
separate component

20. • An agent is a factor whose presence or
absence, excess or deficit is necessary for a
particular disease or injury to occur.
• The environment includes all external factors,
other than the agent, that can influence
health.
• These can be social, physical, or biological
environments.

21. • The social environment encompasses a broad
range of factors, including education,
unemployment, culture regarding diet; and
many other factors pertaining to political, legal,
economic, communications, transportation, and
health care systems.

22. Physical environmental factors are factors like
climate, terrain/related to the land, and
pollution.
• Biological environmental influences include
vectors, humans and plants serving as reservoirs
of infection

23. • From the perspective of epidemiologic triangle,
the host, agent, and environment can coexist
harmoniously.
• Disease and injury occur only when there is
altered equilibrium between them.
• Identify the primary cause and risk factors for
the following diseases.
• Malaria
• Tuberculosis
• HIV/AIDS

24. The Web of Causation
• In this model, effects never depend on single
isolated cause, but rather develop as the result
of chains of causation.
• It was developed especially to enhance
understanding of chronic diseases, such as CVD
disease.
However, it can also be applied to the study of
injury and communicable diseases.

25. • Using this, scientists can diagram how factors
like stress, diet, heredity, and physical activity
relate to the onset of the major cardiovascular
disease: coronary heart disease,
cerebrovascular disease (stroke), and
hypertensive disease.

27. Wheel Model
• Consists of a hub (host or human), which has
genetic make-up as its core, surrounded by the
environment, schematically divided into the
three sectors - biological, social, and physical
• Emphasizes the unity of the interacting factors

28. • The relative size of the different components
of the wheel depend upon the specific disease
problem under consideration
– Hereditary disease - genetic core is relatively
large
– Measles - state of immunity of the host &
biological sector of the environment is large

30. Similarity with web of causation
• identifies multiple etiologic factors of disease
without emphasizing the agent of disease
Difference with the web of causation
• Separately delineates host and environmental
factors, a distinction useful for epidemiological
analyses

31. NATURAL HISTORY OF DISEASE
DEFINITION:
The “natural history of disease” refers to the
progression of disease process in an individual
over time, in the absence of intervention.
Each disease has its own life history, and thus,
any general formulation of this process is
arbitrary.

32. However, it is useful to develop a schematic
picture of the natural history of diseases
 a frame work within which to understand and
plan intervention measures including
prevention and control of diseases.

33. • The process begins with exposure to the
causative agent capable of causing disease.
• Without medical intervention, the process
ends with recovery, disability, or death.

34. There are four stages in the natural history of a
disease.
These are:
1.Stage of susceptibility
2.Stage of pre-symptomatic (sub-clinical) disease
3.Stage of clinical disease and
4.Stage of disability or death

36. 1.Stage of susceptibility
In this stage, disease has not yet developed,
but the groundwork has been laid by the
presence of factors that favor its occurrence.
Examples:
• A person practicing casual and unprotected
sex has a high risk of getting HIV infection.
• An unvaccinated child is susceptible to
measles.
• High cholesterol level increases the risk of
coronary heart disease

37. 2. Stage of Pre-symptomatic (sub-clinical)
disease
In this stage there is no manifest disease
but pathogenic changes have started to occur.
There are no detectable signs or symptoms.
The disease can only be detected through
special tests.

38. Examples:
• Detection of antibodies against HIV in an
apparently healthy person.
• Ova of intestinal parasite in the stool of
apparently healthy children.
• The pre-symptomatic (sub-clinical) stage may
lead to the clinical stage, or may sometimes
end in recovery without development of any
signs or symptoms.

39. 3. The Clinical stage
• By this stage the person has developed signs
and symptoms of the disease.
• The clinical stage of different diseases differs
in duration, severity and outcome.
• The outcomes of this stage may be recovery,
disability or death.

40. 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 becoming disabled
for the rest of their lives.
• Rabies has a relatively short but severe clinical
stage and almost always results in death.
• HIV/ AIDS has a relatively longer clinical stage
and eventually results in death.

41. 4. Stage of disability or death
• Some diseases run their course and then resolve
completely either spontaneously or by
treatment.
• In others the disease may result in a residual
defect, leaving the person disabled for a short or
longer duration.
• Still, other diseases will end in death.
• Disability is limitation of a person's activities
including his role as a parent, wage earner, etc…

42. Examples:
• Trachoma may cause blindness
• Meningitis may result in blindness or deafness

44. Levels of prevention
• Disease prevention means to interrupt or slow
the progression of disease.
• Therefore, the aim is to push back the level of
detection and intervention to the precursors and
risk factors of disease.
• Epidemiology plays a central role in disease
prevention by identifying those modifiable causes.
• The main purpose of investigating the
epidemiology of diseases is to learn how to prevent
and control them.

45. 1. Primordial level of prevention:
• Existence of underlying conditions leading to
causation
• The aim is to avoid the emergence and
establishment of the social, economic, and
cultural patterns of living that are known to
contribute to an elevated risk of disease.
• Example: avoiding smoking, environmental
pollution, heavy drinking

46. 2. Primary prevention:
• The causative agent exists but the aim is to
prevent the development of disease
• It is promoting health, preventing exposure
and preventing disease.
• It keeps the disease process from becoming
established by eliminating causes of disease or
increasing resistance to disease.
• Example: immunization- Measles, polio
• It has 3 components: health promotion,
prevention of exposure, and prevention of
disease.

47. 1. Health promotion: includes general non-specific
interventions that enhance health and the body’s
ability to resist disease; such as:
• Improvement of socio-economic status via well
paid jobs,
• Education and vocational training,
• adequate housing and clothing.
• Emotional and social support and relief of stress,
etc.
• In general it implies healthier and happier life

48. 2. Prevention of exposure
is the avoidance of factors which may cause
disease if an individual is exposed to them
includes provision of safe and adequate water;
proper excreta disposal; vector control; safe
home, school, and street environments

49. 3. Prevention of disease
is the prevention of the disease development
after the individual has become exposed to
the disease causal factors.
This occurs between exposure and biological
onset.
• Involves activities such as active and passive
immunization

50. 3. Secondary Prevention:
is a type of prevention after biological onset of
diseases and before permanent damage by
the disease.
• The objective is to stop or slow the
progression of disease so as to prevent or limit
permanent damage, through early detection
and treatment of disease.

51. We attempt to:
• prevent complications,
• limit disability, and
• Reverse communicability of infectious
diseases.

52. • The common practice called screening aims to
early detection of diseases in apparently healthy
people.
• This is mainly a curative service.
• Other examples of secondary prevention can be;
Early detection and treatment of persons with
infectious diseases,
 prevention of the invasive stages of breast
cancer and
 Prevention of tertiary or congenital syphilis

53. 4. Tertiary Prevention: is a type of prevention
after the disease has already occurred and left
residual damage.
• This consists of limitation of disability and
rehabilitation.
• Rehabilitation refers to the retaining of
remaining functions for maximal effectiveness.
• Limitation of disability helps to limit or stop
the damage and impact of damage.
• The impact can be physical, psychological,
social (e.g., social stigma) and financial.

54. THE INFECTIOUS DISEASE PROCESS
Communicable diseases
• illnesses due to specific infectious agents or
its toxic products
• arise from transmission of agents or toxic
products by direct or indirect mode of
transmission through an intermediate host,
vector or inanimate environment

55. Components of infectious disease process
• Infectious diseases result from the interaction
of infectious agent, susceptible host/reservoir
and environment that brings the host and the
agent together.
Agent: is an infectious micro-organism- virus,
bacteria, parasite, or other microbe.
Host: Host factors influence individual's
exposure, susceptibility or response to a
causative agent.

56. • e.g.- age, sex, race, socioeconomic status, and
behaviors (smoking, drug abuse, lifestyle,
sexual practices and contraception, eating
habits) affect exposure.

57. Environment: are extrinsic factors which affect the
agent and the opportunity for exposure.
• Physical factors - geology, climate,
• physical surrounding (maternal waiting home,
hospital);
• biologic factors such as insects that transmit the
agent;
• socioeconomic factors such as crowding,
sanitation, and the availability of health services

58. Chain of Infection: A model used to understand
the infection process is called the chain of
infection.
• It is logical sequence of factors or links of a
chain essential to the development of
infectious agent and to propagation of disease.
• Each link must be present and in sequential
order for an infection to occur.

59. • Understanding the characteristics of each link
provides with methods to prevent the spread
of infection.
• Sometimes the chain of infection is referred
as the transmission cycle.

60. Components of Chain of Infection
- six components
1. Causative Agent
2. Reservoir host
3. Portal of exit
4. Mode of transmission
5. Portal of entry
6. Susceptible host

61. I. THE AGENT
• This ranges from viral particles to complex multi-
cellular organisms.
• Host agent interaction is characterized by:
a. Infectivity: the ability of an agent to produce
infection (to invade & multiply in a host).
• It is from exposure to infection.
• = (Number of infected)/ (number of susceptible and
exposed) x 100

62. b. Pathogenecity: the ability to produce clinically
apparent infection/disease.
• It is from infection to disease.
• = (Number of clinical cases)/ (number of subclinical
cases)
c. Virulence: the proportion of clinical cases resulting
in severe clinical disease(from disease to outcome)
• = (Number of fatal cases)/ (total number of cases)
d. Immunogenicity: infection’s ability to produce
specific immunity.

64. Factors affecting disease development
1. Strain of the causative agent
2. Dose of agent
3. Route of infection
4. Host age
5. Host nutritional status
6. Host immune response

65. Infectious agent may bring about pathogenic
effects through different mechanisms like:
• Direct tissue invasion
• Production of toxin
• Immunologic enhancement of allergic
reaction
• Enhancement of host susceptibility to drugs
• Immune suppression

66. II. RESORVOIR
• The reservoir of an agent is an organism or
habitat in which an infectious agent normally
lives, grows, and multiplies.
E.g. humans, animals, plants and other inanimate
objects
As a general rule, the greater the number of
different reservoirs for a given disease, the
greater the difficulty in controlling that disease.
(e.g., malaria)
• The case of Typhoid Marry (Marry Malon)

67. • A person who does not have apparent clinical
disease, but is a potential source of infection
to others people is known as a carrier.

68. • Typhoid Marry [Mary Malon] was a carrier of
Salmonella typhi who worked as a cook in
New York City, in different households over
many years.
• She was considered to have caused at least
10 outbreaks of Typhoid fever in New York City
with several deaths.
She was the first known case of a carrier.

69. Carriers can be classified as:
1. Asymptomatic carriers: transmitting disease
without ever showing signs and symptoms of
the disease.
• e.g. polio in 95% of cases, amoebiasis, viral
hepatitis-A in 67- 95%, meningococcus ,etc.

70. 2. Incubatory carriers: transmitting disease
during incubation period
i.e. from first shedding of the agent until the
clinical onset or before onset of symptoms or
manifestations of disease
• e.g. Measles, chicken pox, mumps, viral
hepatitis, AIDS, rabies.

71. 3. Convalescent carriers: transmitting disease
during convalescence period i.e. from after the
time of recovery to when shedding stops.
• E.g. Typhoid fever (about 10% after treatment
infectious for about 3 months), Diphtheria,
Hepatitis B virus
4. Chronic carriers: transmitting disease for a
long period of time, or even indefinitely.
• e.g. viral hepatitis B, typhoid fever

73. Importance of carriers
1. Their Number- carriers may outnumber cases-
constitute a significant number of reservoirs
2. Difficulty in recognition/detectability-
carriers don’t know that they are infected
3. Mobility- carriers are more freely so that have
more contacts, but cases are restricted/bed
ridden
5. Chronicity- carriers re-introduce infection and
contribute to endemicity

74. III. PORT OF EXIT
• Port of exit is the way the infectious agent
leaves the reservoir.
• Possible ports of exit - all body secretions and
discharges: saliva, tear, breast milk, vaginal
and cervical discharges, excretions (urine,
faeces), blood, and tissues.
• semen, pus, exudates from wounds are the
usual portals of exit.
• Respiratory: through exhalation, talking and
coughing:
• Skin and mucus membranes

75. IV. MODE OF TRANSIMISSION
• it is the mechanism by which an infective agent exits
from a reservoir host and enters into a susceptible
host.
two major modes:
1. Direct Transmission- immediate transfer of the agent
from a reservoir to a susceptible host by direct
contact or droplet spread.
Direct contact
• Touching
• Kissing
• Sexual intercourse
• Biting
• Passage through birth canal
• Blood transfusion

76. Direct projection of saliva droplets by expiratory
activities, coughing, sneezing, spitting, talking and
singing.
 usually limited to a distance of one
meter or less.
• Trans-placental, maternal to fetus with
passage through the placenta

77. 2. Indirect Transmission- an agent is carried from
reservoir to a susceptible host by:
- suspended air particles or
-animate (vector-mosquitoes, fleas, ticks...) or
-inanimate (vehicle-food, water, biologic products,
fomites) intermediaries.

78. It can be classified as:
A. Vehicle-born: transmission occurs through
indirect contact with inanimate objects (like
surgical instruments, iv fluid, towels, toys)
and contaminated food, or water.

79. B. Vector-borne: the infectious agent is carried from
reservoir to a susceptible host by an arthropod.
biological or mechanical.
• Biological vector: if the agent multiplies in the
vector before transmission.
E.g. -malaria by the anopheles mosquitoes
-Typhus by ticks or lice
• Mechanical vector: if the agent is carried by the
legs or proboscis. E.g. trachoma by flies

80. C. Airborne: which may occur by dust or droplet
nuclei. e.g. tuberculosis
V. PORT OF ENTRY: the site where an agent enters a
susceptible host. examples
– Nasal mucosa--------------common cold
– Respiratory mucosa ------tuberculosis
– Vaginal mucosa -----------sexual transmission
diseases
– Skin--------------------------hookworm
– Injury site-------------------tetanus

81. VI. HOST
A person lacking sufficient resistance to a particular
pathogenic agent to prevent disease if exposed.
• The susceptible human host can be seen at the
individual level and at the community level.
• At the individual level: it is the interaction between
genetic and environmental factors.
e.g. -Genetic factors: sex, blood type, ethnicity etc
-Environmental factors: immunity acquired as a
result of past infection.

82. • At the community level: host resistance at a
community level is known as herd immunity.
• Herd immunity is the resistance of a
community (group) to invasion and spread of an
infectious agent, based on a high proportion of
individuals in a community.
• The high proportion of immune individuals
prevents transmission by decreasing the
probability of contact between reservoirs and
susceptible hosts.

83. Herd immunity operates best when there is:
1. A single reservoir (the human host),
2. Direct transmission,
3. Total immunity,
4. No shedding of the agent by immune hosts
(no carrier stage),
5. A uniform distribution of immunes, and
6. No overcrowding

84. TIME COURSE OF AN INFECTIOUS DISEASE
• Pre Patent Period: the time interval between
infection (biological onset) and first shedding
(the point at which the infection can first be
detected), as measured by the time of first
shedding of the agent.

85. • e.g. The agent can shed into the blood stream,
where it can be picked up by vectors or in
blood transfusion (malaria) or through other
ports of exit (faeces, body secretions, etc).
• In some conditions, like the AIDS, it is the so
called "window period".

86. • Incubation period: the time interval between
infection or biological onset and first clinical
manifestation (clinical onset) of the disease.
• Communicable period: the time interval
during which the agent is being shed by the
host.
• It is the period during which an infected host
can transmit the infection to others.

87. Latent period: the time interval between
recovery and relapse in clinical disease as in the
case of malaria and epidemic typhus.
Convalescent period: between recovery and
time when shedding stops
Generation period: between exposure/infection
and maximum communicability of exposed host

88. Principles of Communicable Diseases Control
Defn. :- reduction of incidence and prevalence
of communicable diseases to a level where it
can not be a major public health problem.

89. Three broad areas of prevention and control:-
1. Attacking the source (reservoir) of infection.
 That is to reduce the number of infective
organisms.
• This is done by: -
–Treatment of cases and carriers through mass
treatment, as in typhoid fever, schistosomiasis,
tuberculosis.
–Isolation: separation of infected persons for a
period of communicability.

90. N.B. Isolation is indicated when
diseases:
have high mortality and morbidity.
have high infectivity.
c. Quarantine: limitation of movement of person
or animal who has been exposed to infectious
disease for a maximum incubation period for
the disease

91. D. Reservoir control by mass vaccination
to cattle and sheep and killing &
burning infected animals (rabies,
anthrax).
E. Active surveillance of contacts
F. Effective reporting system

92. 2. Interrupting the chain of transmission: is control
of modes of transmission from reservoir to the
new host.
A. Environnemental sanitation: - intestinal parasites
control.
B. Personal hygiene: - trachoma and scabies control.
C. Vector control (mosquito and snails control) –
malaria and schistosomiasis.
D. Disinfection and sterilization: - purification of
potable water, pasteurization of milk, disinfection
of air.

93. 3.Reducing host susceptibility:
A. Immunization - herd immunity
B. Better and improved nutrition
C. Health education
D. Chemoprophylaxis:-malaria, meningitis,
tuberculosis
E. Person protection: - mosquito nets, clothing,
repellents, shoes, etc.

94. • THANK YOU
• ANY QUESTIONS