Epid II.pptx

Communicable disease epidemiology
1

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
• Models of infectious diseases
2

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
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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
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Dynamics of Infectious Disease Transmission
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Infectious Disease Transmission
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Reservoir
Portal of exit
Agent
Portal of
entry
Susceptible
host
Person-to-
person
transmission

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)
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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
 Read about Koch's Postulates
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2. The ecological approach – “Multiple causation"
For infectious diseases to occur there are three essential factors,
1.The etiologic agent; necessary cause
2. Suitable environment for spread and growth of the agent
3. Susceptible host to invade, multiply and produce disease
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.
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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.
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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
• Etiology of disease: All factors that contribute to the
occurrence of a disease. They are related to the agent, host
and environment.
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COMMUNICABLE DISEASE …
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.
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• 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
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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,
• sufficient-component cause model, …
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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
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Epidemiologic Triad
Interaction leads to disease occurence 16

EPIDEMIOLOGIC TRIANGLE or TRIAD (BALANCE BEAM)
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• 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.
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• 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
• 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
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• 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
• Amoebiasis
• Measles
• Common cold
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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.
• 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.
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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
• 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
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Host
(Man)
Biologic Social
env’t env’t
Genetic
core
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
Physical env’t
Fig. - The wheel model of human-environment interactions
The Wheel Model
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Natural history of diseases:
• The natural history of disease refers to the progression of a
disease process in an individual over time, in the absence of
intervention.
• The process begins with exposure to the causative agent
capable of causing disease.
• Without medical intervention the process ends with:-
• recovery,
• disability, or
• death.
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There are four stages in the natural history of disease.
These are:-
1. Stage of susceptibility
2. Stage of subclinical disease
3. Stage of clinical disease
4. Stage of disability or death
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Dynamics of disease infectiousness at the
individual level
Times (days)
Clinical
onset
Incubation period
Time of
infection
Resolution
Relapse
Symptomatic period
Susceptible
• immune
• carrier
• dead
• recovered
Onset of
infectiousness
Latent period
End of
infectiousness
infectious period
Susceptible
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1. Stage of susceptibility
In this stage, disease has not yet developed, but the ground
work has been laid by the presence of factors that favor its
occurrence.
•Presence of factors
•No disease
Example: An unvaccinated child is susceptible to measles.
High cholesterol level increases the risk of CAD
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2. Stage of subclinical (pre symptomatic) disease
-There is no manifestations of disease
- but pathogenic changes have started to occur.
-There are no detectable signs or symptoms.
-The disease can only be detected through special tests.
Example: - Detection of antibodies against HIV in apparently
healthy person.
• This Stage may lead to the clinical stage, or may sometimes
end in recovery without development of any signs or
symptoms
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3. Stage of clinical disease
• By this stage the persons will developed signs and symptoms
of the disease.
• The clinical stage of different diseases differs:
- in duration,
-severity, and
-outcomes.
• The outcomes of this stage may be recovery, disability or
death.
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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.
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4. Stage of recovery, 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 to 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…
• e.g. - Trachoma may cause blindness
- Meningitis may result in blindness or deafness.
- Meningitis may also result in death.
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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.
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1. Primordial level of prevention: before being infected
-Sometimes termed as Health promotion:
-Existence of underlying conditions leading to causation
• Aim: to avoid the emergence and establishment risk factors
(social, economic, and cultural patterns of living that are
known to contribute to an elevated risk of disease).
• The main intervention is through individual and mass
education
• Example: avoiding smoking, environmental pollution, heavy
drinking
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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 development.
 It keeps the disease process from becoming established by
eliminating causes of disease or increasing resistance to
disease.
 Primary prevention may be accomplished by measures
designed to general health and well-being, and quality of life of
people promote or by specific protective measures.
 It has 3 components:
 health promotion,
 prevention of exposure, and
 prevention of disease. 36

I. Health promotion: Improvement of socioeconomic status,
provision of adequate food, housing, clothing, and education
II. Prevention of further exposure:- is the avoidance of
additional risk factors which may cause disease development
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III. Prevention of disease:- is the prevention of disease
development after the individual has become exposed to
the disease causing factors (before biologic onset)
-This is when the intervention aims to prevent initiation of
disease
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3. Secondary prevention:- Early stage of disease.
The objective is to stop or slow the progression of disease so
as to prevent or limit permanent damage.
e.g. Early detection & treatment of cases of tuberculosis & STD
• It is carried out before the person is permanently damaged.
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4. Tertiary prevention
- Late stage of disease: treatment & rehabilitation
- Is targeted towards people with permanent damage or
disability.
-it is needed in some diseases because primary and secondary
preventions have failed, and in others because primary and
secondary prevention are not effective.
-The aim is to prevent severe disability and death.
E.g.: Leprosy
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It has two objectives:
1. Treatment to prevent further disability or death and
2. To limit the physical, psychological, social, and financial impact
of disability, thereby improving the quality of life.
This can be done through rehabilitation, which is the
retraining of the remaining functions for maximal
effectiveness.
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THE INFECTIOUS DISEASE PROCESS
Communicable diseases
 illnesses due to specific infectious agents or its toxic products
by direct or indirect mode of transmission through intermediate
host, vector or inanimate environment
43

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
e.g.- age, sex, race, socioeconomic status, and behaviors (smoking,
drug abuse, lifestyle, sexual practices and contraception, eating
habits) affect exposure
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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
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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.
• 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.
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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
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I. THE AGENT
-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
b. Pathogenecity: the ability to produce clinically apparent
infection/disease.
- It is from infection to disease.
= (Number of clinical cases)/ (number of subclinical cases)
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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.
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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
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• 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
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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
• A person who does not have apparent clinical disease, but is a
potential source of infection to others people is known as a
carrier.
The case of Typhoid Marry (Marry Malon)
52

• 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 and more than 50 deaths of Typhoid fever in New
York City with several deaths. She was the first known case of
a carrier.
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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.
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.
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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
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Time course of disease progression
56

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
4. Chronicity- carriers re-introduce infection and contribute
to endemicity
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III. PORT OF EXIT
- 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.
58

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.
 Touching
 Trans-placental
 Kissing
 Sexual intercourse
 Biting
 Blood transfusion
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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.
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.
60

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
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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
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VI. HOST
• Are people who had no form of immunity the disease and
were therefore at-risk of infection
• 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.
63

• 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.
64

Herd immunity operates best when there is:
1. A single reservoir (the human host),
2. There is only direct transmission,
3. No shedding of the agent by immune hosts (no carrier
stage),
4. A uniform distribution of immunes, and
5. No overcrowding
65

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.
• 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".
67

• Incubation period: the time interval between time of
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 ( from first shedding to last
shedding).
• It is the period during which an infected host can transmit
the infection to others.
68

• 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/time: between infection and maximum
communicability of exposed host regardless of whether the
disease apparent or not apparent
• Most of the time, the GT is ≈ incubation period (IP)
69

Conti….
• Matching
1. the time interval between recovery and relapse in
clinical disease= Latent period:
2. between recovery and time when shedding stops=
Convalescent period
3. between infection and maximum communicability
= Generation period/time
4. the time interval between time of infection or
biological onset and first clinical manifestation
(clinical onset) of the disease= Incubation period
5. the time interval during which the agent is being
shed by the host ( from first shedding to last 70

Spread of disease from person to person depends on:
Generation time
– between infection and maximum communicability of
exposed host regardless of whether the disease apparent or
not
– Most of the time, the GT is ≈ incubation period (IP)
Herd immunity
Secondary attack rate
71

Secondary attack rate
• It is an important measure of spread of disease among
contacts of an index case
• Index case: The case that brings a household or any
other group (community) to the attention of the health
authority
• Often helpful to estimate the spread of disease in a
family, dormitory or other groups
72

73
100
x
cases
index
the
with
contacts
of
Number
Total
period
the
during
cases
index
of
contacts
Among
cases
New
SAR 
It is an indirect measure of the infectiousness of the
agent

Models of infectious diseases
1. A generic disease model
2. The Susceptible-Infectious and Recovery (SIR)
model
3. Basic Reproductive Number
74

A general disease model
Susceptible
host
TIME
Incubation period
Death
Recovery
Exposure Onset
Latent/dormant Infectious Non-infectious
Infection
No infection
Clinical disease
75

Cont...
Susceptible
host
TIME
Death
Recovery
Infection
No infection
Clinical
disease
76

Cont...
Susceptible Infected
Recovered
Boxes
•Susceptible  ‘at risk’
•Infected  with a disease
•Recovered  ‘not at risk’ for
short or long time
Arrows
Shows movement from one state to
another
77

Cont...
• Disease control
– vaccination
• shifts individuals from ‘susceptible’ to ‘recovered’ state
• for some diseases this shift is not permanent
– therapy increases the rate of flow out of the
infected state
• permanent cure moves the individual from the infected
to the recovered state
• temporary cure moves the individual from the infected
to the susceptible state
78

Vaccination
S I
R
•Vaccination
•Shifts individuals from ‘susceptible’ to ‘recovered’
state
•for some diseases this shift is not permanent
79

Therapy producing
temporary cure
S I
R
Therapy increases the rate of flow out of the infected state
Temporary cure moves the individual from the infected to the susceptible
state
80

Therapy producing
permanent cure
S I
R
Therapy increases the rate of flow out of the infected state
Permanent cure moves the individual from the infected to
the recovered state
81

Education
S I
R
Education
Through behavioural change, it reduces shifting from
susceptible to infected
82

SIR model
• Susceptible-infected-recovered (SIR) model describes
spreading of an epidemic in which a susceptible
individual becomes spontaneously immune or
recovered after being infected by a contagious disease.
• S- never been infected and able to catch the disease
-Once infected categorized as I
83

• I- are infected and can spread the disease to
susceptible individuals
The time they spend in the infected compartment is the
infectious period, after which they enter the recovered
compartment.
• R – are assumed to be immune for life
84

Basic Reproductive number (R0)
• Basic reproductive number is denoted by R0
Assumption: all contacts are susceptible to infection
R0 (pronounced “r-nought”) is defined as the average
expected number of people that an infected individual
will infect during his or her period of infectiousness,
assuming that everyone in the population is
susceptible to the disease.
85

Cont’d…
• E.g. if R0 = 9 for measles in a population, then one
person with measles introduced into that population
can be expected to produce nine secondary infections
before recovering, if no one is immune to the disease.
86

Cont’d…
• Basic reproductive number, R0
– for an epidemic to occur, R0 must be > 1
– if R0 < 1, an average case will not be reproduce itself, so
an epidemic will not occur
– the larger the value of R0, the harder it is to control the
epidemic
– a single case can produce more than 1 infective cases,
i.e. small cluster of cases but unlikely to be self
sustaining epidemic
– R0 = 1 the disease is endemic
87

Cont’d…
– R0 is a composite of three important aspects of
infectious diseases:
• the proportion of susceptible contacts, C
• transmission probability P
• duration of infectiousness d.
• The basic reproductive rate is affected by several factors
including the
– duration of infectivity of affected patients,
– the infectiousness of the organism, and
– the number of susceptible people in the population
that the affected patients are in contact with.
88
R0
= Cx px d

IV. Effective reproductive number
• RO usually assumes all contacts are susceptible
• In reality, there are some people immune of the
disease under study
• In these circumstances, the number of new cases
infected persons (R) are below the RO
• We call this reproductive number effective
reproductive number and is denoted by R.
89

Cont…
• Effective reproductive number then will be product of
R0 and proportion of susceptible people in the
community.
(M = proportion of susceptible)
• Example:
– let R0 of measles is 9, and half the population is not
immunized, then R = 9 x ½= 4.5.
– A case of measles will infect about 4.5 new cases before
getting recovery in a population of 50% immune.
R = R0 x M
90

Cont…
• If proportion of susceptible are very low, the probability of
contact with an index case will be low (transmission may not
occur).
• For simple models and a 100%-effective vaccine, the
proportion of the population that needs to be vaccinated to
prevent sustained spread of the infection is given by 1 − 1/R0.
• If immunization gives complete/life long immunity among all
the immunized, and if ‘f’ proportion of individuals are
immunized, proportion of susceptible will be (1-f)
• R = R0 x (1-f)
91

• To eliminate the disease, R should be below 1
• i.e. R = R0 x (1-f) <1; and
• Since Ro is an average, even when Ro is <1
92

Cont….
• Example
• Let R0 for measles be 9.0, proportion of immunization that is
needed to stop transmission of the disease in the community
will be;
• f = 1- 1/R0; = 1 – 1/9 = 0.89
• If we immunize 89% of the population we can stop
transmission of illness in a community, and is called herd
immunity.
• Herd immunity: when a significant proportion of the
population is vaccinated and this provides protection for
unprotected individuals
93

• The larger the number of people who are vaccinated in
a population, the lower the likelihood that a
susceptible (unvaccinated) person will come into
contact with the infection
• In this case it is more difficult for diseases to spread
between individuals
94

Cont…
Summary:
• an epidemic is unlikely to occur if the initial proportion
of the population susceptible, (1-f), is less than 1/R0
• if (1-f) are susceptible, f are immune, thus the critical
proportion of individuals to be vaccinated to achieve
herd immunity is 1 - 1/R0
95

Values of R0 of well-known infectious diseases
Disease Transmission R0
HIV/AIDS Sexual contact 2-5
Diphtheria Saliva 6-7
Influenza Airborne droplet 2-3
Measles Airborne 12-18
Mumps Airborne droplet 4-7
Pertussis Airborne droplet 12-17
Polio Fecal-oral route 5-7
Rubella Airborne droplet 5-7
SARS Airborne droplet 2-5
Smallpox Social contact 6-7
Ebola Body fluids 1.5-2.5
96

• So knowing the R0 for an infectious disease outbreak
can be really helpful if we are implementing control
measures
– vaccination for meningococcal disease
– contingency planning for an influenza epidemic
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Epid II.pptx

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Epid II.pptx