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1. Communicable Disease Control
By
Jemal Y. (BSc, MPH)
May 2023
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By Jemal Y.(2023)

2. Course objectives:
• Up on completion of the course, students will be able
to:
Describe communicable disease causation; agent, host
and environment interaction and the web of causation.
Discuses mechanisms of disease preventions; primary,
secondary and tertiary prevention methods.
Describe methods of surveillance; active and passive
surveillance mechanisms, Integrated Disease
Surveillance System focusing from the perispect of
developing countries.
Identify steps in epidemic/outbreak control
Discuss important communicable diseases in Ethiopia;
important public health diseases in Ethiopia.
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By Jemal Y.(2023)

3. Introduction
• Communicable disease - is an illness or disease due
to specific infectious agent or its toxic products
that arises through transmission of that agent or
its products from an infected persons, animal , or
inanimate source to susceptible host.
• Communicable diseases pose a major threat to public
health and are significant concern to community
health.
• Their public health importance ( significance ) in
terms of human suffering , disability and death is
compounded by the considerable toll they take
on economic growth and development .
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By Jemal Y.(2023)

4. Introduction cont…
• Though the burden of Non-communicable is
showing increasing trend over the last few
decades, communicable diseases remain the
major proportion of disease burden globally.
• Communicable diseases are the leading disease
burdens in developing world.
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5. • The main reasons why communicable disease s are becoming
major public health important are:-
a) Microbial agents of communicable are :-
-Dynamic –use different methods to react and behave
- Resilient –able to feel better quickly after unpleasant events
-Well adapted to exploit opportunities for change and
spread.
b) Its important on economical growth and development
due to human:-
-Suffering
-Death
-Disability
c) Difficulty of controlling many important diseases due to either
of:-
-Luck of vaccine & therapeutic drugs
-Reduced effectiveness of existing drugs
-Spread of drug resistant microbes.
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6. Cont…
• Occur in epidemic forms, and can be sudden and
major public health problems like Ebola.
• The problem is exacerbated by:
– Poor socio-economic status
– Poor personal and environmental hygiene
– Inadequate health service coverage, etc.
• Epidemiological transition
– Change in demographic characteristics of people
– Emergence of antibiotic resistant strains of microbes
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7. Natural History of Disease
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Stage of
Susceptibility
Stage of
Subclinical Disease
Stage of
Clinical Disease
Stage of Recovery,
Disability or Death
Exposure
Pathologic
Changes
Onset of
Symptoms
Usual Time of
Diagnosis
• It refers to the progression of a disease
process in an individual over time, in the
absence of intervention
Natural History of a Disease
By Jemal Y.(2023)

8. Possible outcomes after exposure to
an infectious agent
Exposure
No
infection
Clinical
infection
Subclinical
infection
Death Immunity Carriage Non-immunity
Carriage
By Jemal Y.(2023)
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9. Dynamics of Disease and
Infectiousness
Latent period Infectious period Non-infectious period
Incubation period Clinical disease Recovery
Infection
Time
Onset of
symptoms
Resolution
of symptoms
By Jemal Y.(2023)
Period Between Exposure
to Infectiousness
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10. Relationships Between Time Periods
First patient
Second patient
Incubation period
Latent period Infectious period
Clinical disease
Infection
Incubation period
Latent period Infectious period
Clinical disease
Time
Transmission
Serial interval
or generation time
Transmission
By Jemal Y.(2023)
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11. Cont…
• Begins by exposure to a causative agent
capable of causing disease.
• Without intervention, the process ends
with recovery, disability or death.
• The course can be halted at any time in the
progression by intervention, host factors
and other influences.
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12. • Eventually the host becomes non-
infectious by,
–Clearing the infection, possibly by
developing immunity
–Therapeutic intervention, or
–Death.
• The host may become non-clinically ill
while still harboring the microbe (also
called carrier).
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By Jemal Y.(2023)
Cont…

13. Spectrum of Illness
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Inapparent
Infection
Clinical Illness with Signs and
Symptoms
Recovery
Death
Severe
Disease
Mild
Disease
No signs or
symptoms

14. Classification of Diseases: Spectrum of
Clinical Severity
Class A – INAPPARENT INFECTION FREQUENT (Eg: TB)
0 Percentage of Infection 100
Class B – CLINICAL DISEASE FREQUENT; FEW DEATHS (Eg: Measles)
0 Percentage of Infection 100
Class C – INFECTIONS USUALLY FATAL (Eg: Rabies)
0 Percentage of Infection 100
Inapparent Mild Moderate Severe
(Nonfatal)
Fatal
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15. Time Course of Disease&
Communicability
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16. Cont…
The natural history of infectiousness includes:
• Prepatent period: the time interval from infection to
becoming infectious (shedding of the agent).
• Infectious period: the time during which an
infected host could infect another host or vector.
• Incubation period: the time interval from infection
to symptomatic disease.
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17. Iceberg (Pyramid?) Concept of
Infection
EXPOSURE WITHOUT INFECTION
INFECTION WITHOUT
CLINICAL ILLNESS
MILD ILLNESS
SEVERE
DISEASE
DEATH
CLINICAL
DISEASE
SUB CLINICAL
DISEASE
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By Jemal Y.(2023)

18. Iceberg Concept of Infection
(AIDS)
EXPOSURE WITHOUT INFECTION
INFECTION WITHOUT
CLINICAL ILLNESS
MILD ILLNESS
AIDS
DEATH
CLINICAL
DISEASE
SUB CLINICAL
DISEASE
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19. Epidemiologic
Triad
HOST
Model of Disease Causation
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• Infectious diseases result from the interaction between
the infectious agent, host and environment

20. •Agent factors refer to an infectious
microorganism—virus, bacterium, parasite, or
other microbes. They are necessary but not always
sufficient alone to cause disease.
•Host factors are intrinsic factors that influence an
individual’s exposure, susceptibility, or response
to a causative agent.( age, nutrition, race,…
•Environmental factors are extrinsic factors which
affect the agent and the opportunity for exposure.
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Cont…

21. Factors Affecting Disease Causation
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Host Factors Agent Factors
Environmental
Factors
• Age
• Sex
• Previous Disability
• Behavior
• Genetic
Predisposition
• Height
• Weight
• Virulence of
Organism
• Serotypes of
Organism
• Antibiotic Resistance
• Cigarette-tar content
• Type of glass in motor
car windscreen
• Home crowding
• Air pollution
• Workplace hygiene
• Weather
• Water composition
• Food contamination
• Animal contact

22. 1. Epidemiologic triangle and triad (balance beam):
Traditional model of infectious disease causation
Epidemiologic Triangle
HOST
Models of Infectious Disease
Beam Balance
HOST
AGENT
ENVIRONMENT
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By Jemal Y.(2023) 22

23. 2. Sufficient and Necessary
Component Causes model
• In recognition to the multi-factorial nature
of most diseases, different models have
been proposed (mainly for chronic and
noncommunicable diseases).
• The models emphasize that there is no
single cause.
• It is also called multiple causality of
diseases.
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24. Cont…
• Causal pie is one of the models developed
with necessary and sufficient cause
(components).
• Necessary cause: A causal factor whose
presence is required for the occurrence of
the disease.
• Sufficient cause. A causal factor or
collection of factors whose presence is
always followed by the occurrence of the
disease.
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25. Cont…
• Example of three sufficient causes of a disease.
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A
B C
D
A
D E
F
A
B E
F
I III
II

26. Cont…
• Example of three sufficient causes of a disease.
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I III
II
A
D
C
B
A
F
E
B
A
F
E
D

27. Cont…
• Assume that different four causes are operating
for causation of the outcome.
• Without ‘A’, there is no disease. ‘A’ is considered
as necessary cause, but all diseases are not due to
‘A’ alone.
• ‘B’ , ‘D’, ‘E’ and ‘F’ cause disease through two
mechanisms; similarly, others are involved in
one or two of the mechanisms.
• No component cause acts alone, the factors
interact with their complementary factors to
produce disease
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28. 3. Web Model
• It is response to the idea
of non-infectious
diseases – having no
unique agent.
• The different causes of
disease do interact one
another.
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Disease
Factor 1

29. Chain of Infection
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Components of
Chain of Infection
1. Causative Agent
2. Reservoir Host
3. Portal of Exit
4. Mode of Transmission
5. Portal of Entry
6. Susceptible Host

30. Chain of disease transmission…
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31. Susceptible Host
(via portal of entry)
Route / Mode of
Transmission
Reservoir
(via portal of exit)
Agent
Chain of Infection
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Communicable Disease Epidemiology

32. Reservoir
Habitat in which the disease agent normally lives
and multiplies
• Humans
– Symptomatic - Smallpox
– Asymptomatic - HIV
• Animals (Zoonoses)
– Brucellosis, plague, psittacosis
• Environmental
– Soil: Botulism, histoplasmosis, tetanus
– Water: Legionella
Reservoir
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33. Reservoir Versus Source
• Reservoir - Habitat in which the
pathogen normally lives and multiplies
–Humans, animals, environment
• Source – means by which the pathogen
is directly transmitted to humans
–Humans, animals, environment,
insects, food, water, medications,
medical devices, et al.
Reservoir
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34. Portal of Exit
Nose — secretions
Mouth — saliva, sputum
Skin, intact — pustules, lesions
Skin, broken — blood
Breast (females) — milk, secretions
Vagina (females) — secretions
Penis (males) — ejaculate
Anus — feces
Exit
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Communicable Disease Epidemiology

35. Modes of Transmission
• The transfer of an infectious agent from an
infected host (reservoir) to a susceptible
host.
• Direct transmission: Immediate direct
transfer of the agent from a reservoir to a
susceptible host.
• Indirect transmission: Transmission of an
infectious agent to a susceptible host
through the aid of a vehicle, a vector or
suspended air particles.
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By Jemal Y.(2023)
Transmission

36. Modes of Transmission
• Direct
– Contact – Herpes Type I, STIs
– Vertical – rubella (across placenta), GC (birth canal),
HIV (across placenta, breast milk)
– Droplet – smallpox
• Indirect
– Airborne – measles
– Vehicle-borne
• food or water – Salmonella, hepatitis A
• fomites, biologic products – coxsackievirus
– Vectorborne
• Mechanical – Shigella by fly limbs
• Biological – malaria (maturation)
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Transmission

37. Vertical Transmission
• Transplacental
– TORCH Complex
• Toxoplasma
• Other (syphilis, VZV)
• Rubella
• Cytomegalovirus
• Herpes
– Listeria
– Parvovirus B-19
– HIV
• During Birth
– Group B Streptococcus
– Herpes
– Gonorrhea
• Breastfeeding
– HIV
– HTLV-1, HTLV-2
– Others?
Transmission
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38. Transmission
Airborne versus Droplet Transmission
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39. Food-borne, Waterborne Diseases
Hepatitis A?
Transmission
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40. Some Causes of
Food-borne, Waterborne Diseases
Food-borne
• Bacillus cereus
• Campylobacter jejuni
• Clostridium botulinum
• Clostridium perfringens
• Cyclospora cayetanensis
• E. coli O157:H7
• Hepatitis A
• Listeria monocytogenes
• Norovirus
• Salmonella species
• Staphylococcus aureus
• Yersinia enterocolitica
Waterborne
 Entamoeba histolytica
 Cryptosporidium parvum
 Giardia lamblia
 Hepatitis A
 Legionella pneumophila
 Vibrio cholerae
 Parasitic
– Schistosoma
– Dracunculus
– Taenia solium
– et al.
Transmission
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By Jemal Y.(2023)

41. Vehicle-borne Transmission
MMWR, May 16, 2008
Acute Hepatitis C Virus Infections Attributed to Unsafe Injection Practices at an
Endoscopy Clinic — Nevada, 2007
Transmission
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By Jemal Y.(2023)

42. Vectors and Vector-borne
Diseases
Pathogen Vector Disease
West Nile virus mosquito WNV
encephalitis
Yersinia pestis flea Plague
Rickettsia prowazekii louse, tick Epidemic typhus
Plasmodium falciparum mosquito Malaria
Trypanosoma cruzi tsetse fly Chagas’ disease
Onchocerca volvulus Simulium fly River blindness
Transmission
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By Jemal Y.(2023)

43. Susceptible Host
(via portal of entry)
Portals of Entry
 Eye (conjunctivitis)
 Mouth – food-borne, water-
borne, fecal-oral
 Nose / mouth – airborne,
droplet (tuberculosis)
 Skin (hookworm)
 Genital tract, anus (STIs)
 Bloodstream (hepatitis B, HIV)
Entry
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Communicable Disease Epidemiology

44. Susceptible Host
(via portal of entry)
Route of
Transmission
Reservoir
Agent
Chain of Infection – Control?
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Communicable Disease Epidemiology

45. Levels of Disease Occurrence
Diseases occur in a community at different
levels at a particular time in time:
1. Expected Level (Predictable) and
2. Excess of expected
1. Expected level of occurrence of disease
• Endemic: the usual presence of disease from low
to moderate level. constant presence of disease.
• Hypo/Meso/Hyper-endemic: a persistently
lower or moderate or high level of disease.
• Sporadic: Normally does not occur, but
occasional cases occur at irregular intervals
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46. Cont….
Excess of expected levels
• Epidemic/ Outbreak : An excess
occurrence of disease over expected level
at certain time.
unusual occurrence of case /illness with a frequency in
excess of normal expectancy.
• Pandemic: An epidemic that affects
several countries or continents. (eg
HIV/AIDS)
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47. Levels of Disease Occurrence
(Endemic Vs Epidemic)
Endemic
Epidemic
Number
of
Cases
of
a
Disease
Time
Hyperendemic
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48. Levels of Disease
Sporadic
Endemic
Epidemic
Pandemic
Increasing amount of disease
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49. Infection and Disease Outcome
• Exposure to an infectious agent does not
necessarily lead to infection, and
• An infection does not necessarily lead to
disease
• Infection may remain asymptomatic
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50. Outcomes at each stage of infection
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Exposure
Disease
Outcome
Disease
Infection
Infectiousness Virulence
Pathogenicity
Cont…

51. Cont…
• The progress of an infectious agent and
disease outcome can be quantified as
follows:
1. From exposure to infection
• Infectiousness: the proportion of an
exposed susceptible host who become
infected (measured by infection rate), as:
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52. 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:
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53. 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
By Jemal Y.(2023) 53

54. Levels of Disease Prevention
• It is important to the levels in details for
implementing interventions that prevent
or ameliorate infections.
• It involves the interruption or slowing of
disease progression through appropriate
intervention.
• Epidemiology plays a central role in
disease prevention by identifying
modifiable causes of disease and their risk
factors.
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By Jemal Y.(2023)

55. Cont…
There are several stages during the course of
a disease where we can intervene in order to
control the disease.
Three levels, (Primary, Secondary and
Tertiary)
I. Primary Prevention
The objectives here are to promote health,
prevent exposure, and prevent occurrence of
disease.
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By Jemal Y.(2023)

56. Cont…
A. Health Promotion (Primordial):
• This consists of general non-specific
interventions that enhance health and the
body’s ability to resist disease.
• The improvement of socioeconomic status
through the provision of adequate:
– education,
– affordable and adequate housing and
clothing, etc.
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57. Cont…
B. Prevention of exposure:
• Specific to individual diseases compared to
primordial prevention
Example
– Provision of safe and adequate water, proper
excreta disposal
– Vector control;
– Provision of a safe environment at home
– Use of bed nets
– Consistent use of condom
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58. Cont…
C. Prevention of Disease:
–This is when the intervention aims to
prevent initiation of disease, in persons
who may already be exposed to agent
–An example of intervention, which acts
at this stage is immunization.
–Some times it may be difficult to
differentiate interventions in what form
of prevention they involved.
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59. Cont…
II. 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 and to prevent or
limit permanent damage.
• Strategy at this stage is through early
detection and treatment of disease.
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60. Cont…
III.Tertiary Prevention
• Intervention that acts after permanent
damage has set in, and the objective of
tertiary prevention is to limit the impact of
the damage.
• The impact can be physical, psychological
social (social stigma or avoidance by others),
and financial.
• Strategy at this stage in general is
rehabilitative.
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61. Exercise
1. How do we prevent malaria?
 Primary
 Secondary
 Tertiary
2. How do we prevent STIs?
 Primary
 Secondary
 Tertiary
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