1.
Communicable diseases epidemiology
(Infectious disease cycle)
BY: Agumas Fentahun
(BSC in HO, MPH in Epidemiology)
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2.
Communicable diseases epidemiology
(Infectious disease cycle)
1/26/2023
By: Agumas F.

3.
Communicable Disease (Infectious
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,
animal, or reservoir and transmit to a susceptible
host, either directly or indirectly through an
intermediate plant or animal host, vector, or the
inanimate environment.
1/26/2023
By: Agumas F.

4.
The causes of disease can be classified in to two
1. Primary causes (Etiologic Agent) are the factors which
are necessary for a disease to occur, in whose absence the
disease will not occur
2. Risk factors (contributing, predisposing, or aggravating
factors).
 The etiology of a disease is the sum total of all the factors
(primary causes and risk factors)
1/26/2023
By: Agumas F.

5.
Infectious disease cycle
Infectious diseases result from the interaction of
I. Infectious(Etiologic)agent
II. Susceptible host/reservoir
III. Environment that brings the host and the agent
together.
I. Etiologic Agent: is an infectious micro-organism-
 virus,
 bacteria,
 parasite, or other microbe
1/26/2023
By: Agumas F.

6.
II. Host: Host factors influence individual's exposure,
susceptibility or response to a causative agent.
For example
Age,Sex, Race
 Socioeconomic status
Behaviors (smoking, drug abuse, lifestyle, sexual
practices and contraception, eating habits) affect
exposure.
Infectious disease cycle
1/26/2023
By: Agumas F.

7.
III. Environment: Environmental factors are extrinsic
factors which affect the agent and the opportunity for
exposure.
 Physical ,
 Biologic,
 Socioeconomic factors
Infectious disease cycle
1/26/2023
By: Agumas F.

8.
 Physical factors such as geology, climate, and
physical surrounding (e.g., 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.
1/26/2023
By: Agumas F.

9.
The epidemiologic triad or triangle is the traditional
model of infectious disease causation.
Epidemiologic triad has three components:
 an agent,
 a susceptible host, and
 an environment that brings the host and agent together.
Infectious disease cycle
1/26/2023
By: Agumas F.

10.
Examples of causes of disease by host, agent and
environmental factors.
Host factors Agent factors Environmental
factors
Age
Sex
Previous disability
Behaviour
Genetic
inheritance
Height
Weight
Virulence of
organisms
Serotype of
organisms
Antibiotic
resistance
Cigarette-tar
content
Home
overcrowding
Air pollution
Workplace
Hygiene
Weather
Water composition
Food
contamination
Animal contact
1/26/2023
By: Agumas F.

11.
Chain of Infection
 Infection implies that the agent has achieved entry
and begun to develop or multiply, whether or not the
process leads to disease.
 Chain of infection is a model used to understand the
infection process which has different components
 Understanding the characteristics of each link
provides insight for methods to prevent the spread of
infection.
1/26/2023
By: Agumas F.

12.
Chain of infection
 Components of Chain of Infection
 Causative Agent
 Reservoir host
 Portal of exit
 Mode of transmission
 Portal of entry
 Susceptible host (final link in chain of infection)
1/26/2023
By: Agumas F.

13.
Chain of infection
1. Causative Agent
 Is a particle which range from small viral
particles to complex multi cellular organisms
(parasites)
 Characterized by its
Size
Chemical character
Antigenic makeup
Ability to survive out side the host
Ability to produce toxin
1/26/2023
By: Agumas F.

14.
Chain of infection
Host –agent interaction
 Host –agent interaction is characterized by
Infectivity
Pathogen city
Virulence
Immunogenicity
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15.
Chain of infection
Spectrum of infectious disease
 The range of disease from asymptomatic to
illness with mild to severe or fatal condition
 If many individuals are equally exposed to
infectious agent
 Some may not be infected at all
 Some are infected but develops no or mild
symptoms
 Some may be infected and develop severe
symptoms
 Others may die as a result of their infection
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Chain of infection
Spectrum of infectious disease….
 Terms commonly used to explain the spectrum
of infectious disease are infectivity,
Pathogenicity and virulence
 Infectivity
 refers to the ability of an organism to invade and
infect the host
 Measured based on the proportion of exposed
persons who become infected or
 The ability of the agent to invade and multiply in
the host (the ability to produce infection )
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17.
Chain of infection
Spectrum of infectious disease…
 Pathogenicity
 refers to the ability to cause disease
 Measured by proportion of infected individuals who
develop clinically apparent disease, or
 the ability to produce clinically apparent infection
 Virulence
 Refers to the ability to cause serious complications or
death
 Measured by the proportion of clinically apparent
cases that are develop severe or fatal clinical disease
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18.
Chain of infection
Spectrum of infectious disease…
 Immunogenicity: the infection ability to
produce specific immunity
 The spectrum of disease can be affected by:
strain of the organism, dose of agent, route of
infection, host immunity and influence of
treatment
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chain of infection
Pathogenic mechanisms of disease
 Direct tissue invasion(ex. bacillary dysentery/
shigella species mainly)
 Production of toxin(e.g Tetanus/ clostridium
tetani)
 Immunologic enhancement or allergic reaction
 Persistent /latent infection
 Enhancement of host susceptibility to drugs
 Immune suppression(HIV)
1/26/2023
By: Agumas F.

20.
Chain of infection
2. Reservoirs host
 The reservoir of an agent is the habitat in which
an infectious agent normally lives, grows, and
multiplies.
 Thus , human reservoirs may be persons with
symptomatic illness, or carriers.
 A carrier is a person without apparent disease
who is capable of transmitting the agent to others
1/26/2023
By: Agumas F.

21.
Chain of infection
Some disease of human reservoir are
 Most bacterial and viral diseases
 Most staphylococcal and streptococcal
infections
 STDs , etc
 Some disease transmit from animal to human
beings and they are named as zoonotic
diseases/zoonoses
1/26/2023
By: Agumas F.

22.
Chain of infection
 Asymptomatic carriers : transmit the
infection without ever showing signs of the
disease
 Incubatory carriers : transmit the infection
by shedding the agent before the onset of
clinical manifestations
 Convalescent carriers : transmit the
infection after the time of recovery from the
disease
 Chronic carriers : transmit the infection for a
long period of time, or even indefinitely.
1/26/2023
By: Agumas F.

23.
Chain of infection
 The importance of carriers in the transmission
of disease depends on their:
 1) number
 2) detect ability
 3) mobility
 4) chronicity.
1/26/2023
By: Agumas F.

24.
Chain of infection
3. Portal of exit
 is the way the infectious agent leaves the reservoir host
and these includes all body secretions and discharges
4. Mode of transmission
 is the various mechanisms by which agents are
conveyed to a susceptible host
 Direct Transmission:- immediate transfer of the agent
from a reservoir to a susceptible host by direct contact
or droplet spread.
 Direct contact ,
 Direct projection,
 Transplacental route 1/26/2023
By: Agumas F.

25.
Chain of infection
 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)
intermediaries.
 Example:
⎯ Vehicle-born: food, water, towels,
⎯ Vector-borne: insect animals, ...
⎯ Airborne: dust, droplets
1/26/2023
By: Agumas F.

26.
Chain of infection
5. Portal of entry
Is through which that an infectious agent enters a
susceptible host
For Example:
 Nasal _ Common cold
 Vaginal – Sexually transmitting diseases
 Injury site _ Tetanus
 Skin – Hookworm infection
 Respiratory –Tuberculosis
1/26/2023
By: Agumas F.

27.
Chain of infection
6. Susceptible host
 The chain of infection may be interrupted if the
agent does not find a susceptible host.
1/26/2023
By: Agumas F.

28.
The susceptible human host
 Defn:- A person lacking sufficient resistance
to a particular pathogenic agent to prevent
disease if exposed.
 The concept of host susceptibility or resistance
can be seen at two levels, individual and that
of the population.
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At the individual level
 The state of the individual host at any given
time is the result of interaction of genetic
endowment with the environment over the
entire life span.
Examples:
 Genetic factors: sex, blood type, ethnicity etc.
 Environmental factors: immunity acquired as a
result of past infection
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At the community level
 Host resistance at the community (population)
level is called herd immunity.
 Herd immunity can be defined as the
resistance of a community (group) to invasion
and spread of an infectious agent, based on
the immunity of a high proportion of individuals
in the community.
 This may occur if a high proportion of
individuals in a population is resistant to the
agent.
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31.
At the community level…
 The high proportion of immunes prevents
transmission by highly decreasing the
probability of contact between reservoirs and
susceptible hosts.
 Through such herd immunity, immune persons
limit the spread of the infection to the relatively
few who are susceptible by reducing the
probability of contact between infected and
susceptible persons.
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32.
Conditions under which herd immunity best functions
 Single reservoir (the human host): If there is
other source of infection it can transmit the
infection to susceptible hosts.
 Direct transmission (direct contact or direct
projection): Herd immunity is less effective for
diseases with efficient airborne transmission.
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33.
Conditions…
 No shedding of agents by immune hosts
(no carrier state).
 Uniform distribution of immunes:
Unfortunately, susceptible usually happen to
live in clusters or pockets because of
socioeconomic, religious, or geographic
factors.
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34.
Conditions…
• No overcrowding: Overcrowding also
increases the likelihood of contact between
reservoirs and susceptible hosts.
• Total immunity: Partially immune hosts may
continue to shed the agent, and hence
increase the likelihood of bringing the infection
to susceptible hosts.
• However, these conditions for the operation of
herd immunity are seldom fulfilled.
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35.
Infection versus Disease
 Exposure does not necessarily lead to
infection.
 Infection is defined as the entry and
development or multiplication of an infectious
agent in a host, whether or not this process
results in disease.
 Infection may remain asymptomatic or may
appear as a disease.
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36.
Chain of Infection …
Implications (of chain of infection) for public
health
 provides a basis for determining appropriate
control measures
 Eg: -treating patients with antibiotics
-Decontaminating equipments
-isolation of someone with infection, or counseling
persons to avoid the specific type of contact
associated with transmission
-Using bed nets, gloves, etc
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37.
Time course of an infectious disease
 Pre-patent period
 Incubation period
 Communicable period
 Latent period
1/26/2023
By: Agumas F.

38.
Time course of an infectious disease
 Pre-patent period- the time interval between
biological onset and the time of first shedding
of the agent
 Incubation period- the interval between
biological onset and clinical onset
 Communicable period- the time interval
during the agent is shed by the host
 Latent period –the interval between recovery
and relapse in clinical disease
1/26/2023
By: Agumas F.

39.
1/26/2023
By: Agumas F.

40.
Infection vs disease
 Infection -is the condition in which the causative
agent invaded the host and multiplied in the host
 Disease -is a condition in which clinically apparent
onset of the problem was observed
1/26/2023
By: Agumas F.

41.
Thank you!
1/26/2023
Getasew M.
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