EPIDEMIOLOGY AND EPIDEMIOLOGICAL METHOD

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UNIT-IV
COMMUNITY HEALTH NURSING-I
PRINCIPLES OF EPIDEMIOLOGY AND
EPIDEMIOLOGICAL METHODS
UNIT TOTAL HOURS-10
EPIDEMIOLOGY
EPI upon
DEMO’s people
LOGOS study
1. Epidemiology is a philosophy, a strategy,
a methodology, a way of studying health
problem
2. The study of the frequency, distribution
and determinants of disease”
- - International Epidemiological
Association
Frequency-
Related to rate and ratios.
Distribution-
Related to time, place & Person.
Determinants-
Related to etiology or risk Factors
HISTORY OF EPIDEMIOLOGY

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DEFINITION
Epidemiology is the study of the distribution and
determinants of health-related states or
events in specified populations, and the
application of this study to control of health
problems
Concept of epidemiology
 Epidemiology is a strategy for the study of
factors relating to the etiology, prevention,
and control of disease; to promote health;
and to efficiently allocate efforts and
resources for health promotion,

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maintenance and medical care in human
populations.
 Epidemiology is that field of medical
science which is concerned with the
relationship of various factors and
conditions which determine the frequencies
and distributions of an infectious process, a
disease, or a physiologic state in a human
community.
Terminology
1. Outbreaks/Epidemics
 The occurrence of more cases of
disease than is expected in a given
area over a particular period of
time.
2. Surveillance
 The ongoing systematic
collection, analysis,
interpretation, and dissemination
of health data=Information for
Action!
3. Incidence
 # of New Cases Occurring in a
Given
Population in a Specified Time Period_
Population at Risk in That Time Per
(Speedometer)
4. Prevalence
 # of Cases Existing in a Given
Population at a Single Point in Time__
Population at That Time (Odometer)
5. Endemic-
 The constant presence of disease
within a geographical area.
6. Pandemic-
 An epidemic which spreads from
country to country or over the
whole world.
7. Infection-
 The entry and development of a
disease producing agent in the
body of man or animal.
8. Communicable disease-
 An illness due to a specific
infectious agent or its toxic
products which, under certain
condition, tends to spread among
individuals in community.
9. Incubation periods-
 This is time interval between the
entry of the disease agent in to the
body and the appearance of the
first sign or symptoms of the
disease.
10. Isolation-
 The separation of a person with
infectious disease from contact
with other human beings.
11. Vector-
 Usually an arthropod which
transfers an infectious agent from
an infected person to a healthy
host.
12. Virulence-
 Measure of the severity of
disease.
13. Pathogenicity-
 Ability to cause disease
1. Mortality
“The rate of Death”

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2. Morbidity
“The rate of Disease”
PURPOSE OF EPIDEMIOLOGY
OBJECTIVES OF
EPIDEMIOLOGY
AIMS OF EPIDEMIOLOGY
According to IEA, epidemiology has three main
aims-
1. To describe the distribution and magnitude of
health and disease problem in human
population.
2. To identify etiological factors in the
pathogenesis of disease.
3. To provide the data essential to the planning,
implementation and evaluation of services
for the prevention, control and treatment of
disease
USES OF
EPIDEMIOLOGY
A. Community diagnosis; i.e., what are
the major health problem occurring in a community
B. Establishing the history of a disease
in a population; e.g., identifying the Periodicity of
an infectious disease
3. Describing the natural history of Disease in the
individual; e.g., natural history of HIV
infection in the individual (infection-
acute syndrome-asymptomatic phase-clinical
disease-death)
D. Describing the clinical picture of Disease;
i.e., who gets the disease,
Who dies from the disease, and what the
outcome of the disease is Estimating risk;
e.g., what factors Increase the risk of heart
disease, Automobile accidents, and violence
F. Identifying syndromes and precursors; E.g.,
the relationship of high blood
Pressure to stroke, kidney disease, and Heart
disease
G. Evaluating prevention/intervention
Programs

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H. Investigating epidemics/diseases of
unknown etiology
SCOPE OF EPIDEMIOLOGY
Now epidemiology is concerned with the
systematised study of:
 Whole population in their living and
working environment
 Factors that determine a state of health
and disease
 Pattern of health as well as patterns of
illness
 Mass phenomenon and effect of
diseases or conditions of groups or
individuals ,as small as
 family or as large as a whole
community, nation or group of nations,
 Distribution and causes of human
health problems
 Mutilpe factors of causation
 Measures of prevention and control.
The ultimate aim is to reduce problems and its
consequences in the community by
 Studying the variations in occurrence
and the distribution of communicable
disease in different communities
 Establishing a community diagnosis of
the presence ,nature and the
distribution the community, disorders
through morbidity and mortality rates
along with identifying high risk
population
 Identifying the causes of
communicable disease by defining
various geographic, demographic,
genetic, environmental and social
factors.
 Estimating individual risks and
chances towards disease occurrence in
general or specified segments of
population
 Identifying and describing various
syndromes in the community
 Detecting clinical and subclinical
forms of disorder in communities for
early diagnosis through screening
mechanism
 Studying knowledge,attitudes,beliefs
and practices of communities towards
communicable problems and to
develop intervention programmes
 Investigating epidemic problems and
to evolve appropriate intervention
programmes
 Completing natural history of diseases
affecting general population
 Developing strategies for planning
,organising, implementing ,integrating
and evaluating services in
communities
CONTRIBUTION OF EPIDEMIOLOGY
 Epidemiology makes community
health nursing a dynamic
 It adds methods of hypothesising new
problem solving technique to the
nursing process for practice
 formulates new relationships and new
association between nursing and public
health
 assists nurses to meet changing
community needs
 common language improves inter-
professional communication and trust
CONTRIBUTION OF
EPIDEMIOLOGICAL PERSPECTIVE
FOR THE NURSING
 Epidemiology provides a framework
within which basic science and
behavioural science can be used for
community nursing practice.
 The nursing process extended through
application of epidemiological
methods to describe community needs
and evaluate nursing services
 Public health principle of family is the
unity of society. Prevention and control
of disease and health promotion are
activated and quantifies through
epidemiological approach

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 An Epidemiological perspective
provides a method of extending the
relationship of family problems to
community welfare.
 The Epidemiologic model promotes
understanding the relationship between
the environment and agents that expose
susceptible populations at risk of
impediments to health.
 Epidemiology provides time honoured
method of quantifying nursing
outcomes such as recidivism. Lack of
compliance and activities of daily
living, to promote and improve the
quality of nursing care in the
community.
 Epidemiology helps community health
nurses to identify and investigates the
problems, formulate alternative actions and
implements the prevention and control of
problem and also helps to evaluate the
effectiveness of actions.
 For example, nurses in the community have
an active role in prevention and control of
communicable disease which includes-
participation in early diagnosis and
treatment, notification of certain specific
diseases to the health authority, tracing the
contact, keep them under surveillance,
identify sources of infection and educating
the people in genes.
Hence, the knowledge of epidemiology is
essential for any nurse for her fruitful practice
IMPLICATION OF
EPIDEMIOLOGY
 To study the effects of disease
state in a population over a time
and predict future health needs
 To diagnose the health of the
community
 To evaluate health services
 To estimate the individual risk
from group experiences
 To identify the syndrome
 To complete the clinical picture of
chronic disease and describe their
natural history
 To search for causes of health and
diseases
DUTIES AND
RESPONSIBILITIES OF
EPIDEMIOLOGY NURSE

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Basic Measurement in
Epidemiology
o Morbidity
o Mortality
o Disability
Tools of
Measurement
o Rates
o Ratios
o Proportions
o Relative Risk
o Odds Ratio
RATIO: A fraction in which the numerator
is not part of the denominator.
o Ex: Fetal death ratio: Total no. of fetal
deaths/total no. of live births
o Fetal deaths are not part of live births
PROPORTION: A fraction in which the
numerator is part of the denominator.
Ex: Proportional mortality.
o Most fractions in epidemiology are
proportions.
RATE: A proportion in which change over
time is considered
o - But in practice, the term “ rate” is often
used interchangeably with ratio without
reference to time
o Ex: fetal death rate & fetal death ratio,
maternal mortality rate & maternal
mortality ratio.
o Prevalence: frequency of existing cases
o Incidence: frequency of new cases
o New cases are called incident cases.
Existing cases are called prevalent cases
PREVALENCE RATE:
No. of people with disease at specified time
No. of people in Population at risk at specified time
x 10
n
Point prevalence: Proportion of a population
affected by a disease at a given time.
Period Prevalence: Proportion of individuals in a
specified population at risk who have the disease of
interest over a specified period of time.
Ex: annual prevalence rate.
INCIDENCE RATE
 Incidence Rate is defined as the no. of
NEW cases occurring in a defined
population during a specified time
period.
No. of new cases of specific disease during a given
time period Population at risk during that
period
X 1000

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EPIDEMIOLOGY TRIAD
Disease is the result of forces within a dynamic
system consisting of:
Agent of infection
Host
Environment

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Disease Cycle Intervention
The interaction of host, agent, and
environment makes up the disease cycle.
Although the agent must be present for a
disease to occur, it alone is not a sufficient
cause. The cycle must be completed for the
disease to occur or
Conversely, the cycle must be broken to
control the disease.
HOS
T
AGEN
T
VECTO
R
ENVIRO
NMENT

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FACTORS RESPONSIBLE FOR
THE SPREAD OF DISEASE
 Agent
 Specific living or inanimate objects
that can cause health problems to
hosts.
 Environment
 Is the favorable surroundings and
conditions external to the human or
animal that cause or allow the disease
or allow disease transmission
 Host
 Groups of living organisms (people,
animals, and plants) that, under
certain circumstances, may become
unhealthy
AGENT
 It includes
• biological agents
• Physical agents
• Nutritional agents
• Chemical agents
• Mechanical agents
BIOLOGICAL AGENT
Involves in occurrence of disease
1) Virus(HIV)e.g AIDS
2) Rickettsia(typhus)
3) Fungi(candida)e.g vaginal itching
4) Bacteria(streptococcus)e.g pneumonia
5) Protoza(plasmodium)e.g malaria
Physical agents
COLD
Impact of cold weather
 Frostbite(numbness of skin, skin appears
whitish and waxy)
 Influenza(flu,headache,runny nose )
 Hypothermia(body temp falls below 37)
HEAT
Heat disorders
 Heat cramps(painful muscle contraction
begins after stopping exercise in heat)
 Heat syncope(sudden fainting occurs while
standing in heat for 15 to 20 mints)
 Heat edema(mild swelling of hands and feet)
 Prickly heat/heat rash(small red itching
lesions on skin caused by obstruction of
sweat ducts)
RADIATIONS
 Effect of radiations such as x rays are used for
detection but their excessive use can cause cancer
similarly exposure to UV light can also cause cancer
CHEMICAL AGENTS/INANIMATES
 The chemical agents mostly affected people work
in an industry & exposure to such chemicals lead
to diseases (fumes,alkaloids)
CONTACT WITH SKIN:
 Urticaria
 Itching
THROUGH INHALATION:
 Severe coughing
 Chest pain
 Dyspnea
THROUGH INGESTION:
 Vomiting (by CO poisoning)

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MECHANICAL AGENTS
 Injury
 Accidents
 Machinery
NUTRITIONAL AGENTS
 Deficiency of these agents affecting people of all
genders and ages. They not only cause specific
diseases but effect the quality of life
These are the nutritional agents:
 Vitamins
 Minerals
 Proteins
 Carbohydrates
 Diseases which are caused by deficiency of the
nutritional agents:
 Osteoporosis(by the deficiency of ca)
 Anemia (by deficiency of iron)
 Scurvy (deficiency of vitamin C)
 Marasmus (deficiency of proteins)
 Acidosis ( deficiency of carbohydrates)
Susceptible host
 Host factors
1) Demographic:
Study of human population & how they change & how they
become unhealthy. E.g. age, sex, ethnicity
(common characteristic of a group of people)
2) Genetics/hereditary:
Transmission and variation of inherited characteristic. E.g.
hypertension, diabetes
3) Immunity:
 State of being insuscepectible to something.
 When there is little to no immunity within a
population, the disease spreads quickly
 E.g. measles in children
4) SOCIAL AND ECONIMICAL:
The social & economic factors has a significant effect on
their health and wellbeing. E.g lungs cancer in adults
due to smoking
Environmental factor
Seasons/weather:
Also affect the humane health
E.g. in rainy seasons malaria can occur
Similarly there is cold in winters.
Allergy due to pollens.
DYNAMICS OF DISEASE TRANSMISSION
 Existence of Source of infection or reservoir
is starting point.
DEFINITION OF RESERVOIR:
 Any person , animal, plant , soil in which
infectious agent survives and multiply in
such a way that it can be transmitted.
RESERVOIR
 Human reservoir
 Animal reservoir
 Non living reservoir
Human reservoir
 It may be CASE and CARRIER
Case:
Case is a person who has a particular disease. it can be
identified through signs and symptoms of the disease
, through diagnostic test or physical examination e.g.
patient of TB
Carrier:
Carry the organism of disease. Person may be infected but
not clinical diseased. E.g. hepatitis (in this virus
inactivate for the time being but can be activated at
any stage of life)
Animal reservoir
 Also called zoonoses
 An animal become reservoir when disease
which is transmitted through animal infected
most of the population
 Causative agent of disease survive and
multiply in that animal
 e.g influneza
Non living reservoir
 Includes soil , water etc
 Soil contains bacteria which cause tetnaus
 Water contains micro organism(protoza)
causing different diseases like malaria
dengue….
MODE/ROUTE OF TRANSMISSION

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• Infectious disease can spread in a variety of ways ,
through air, food.
• Through DIRECT & INDIRECT contact with other
person, objects skin and mucous membrane , saliva,
urine , blood and body secretions
• Through contaminated food and water
ROUTES OF TRANSMISSION
DIRECT
Direct contact
Vertical transmission
Droplet infection
Animal bite transmission
Contact with soil
INDIRECT
Airborne
Vehicle borne
Vector borne
Formite borne
Hand borne
DIRECT TRANSMISSION
Immediate transfer of the pathogen or agent from a
host/reservoir to a susceptible host
Can occur through direct physical contact or direct personal
contact such as touching contaminated hands, kissing or sex
Direct person-to-person contact with the skin or bodily fluids
of a diseased person. Examples are dysentery, boils, and
several airborne diseases
Mucus-to-mucus contact by kissing or sexual intercourse.
Examples include sexually transmitted diseases (STDs),
infectious mononucleosis, and hepatitis B
Direct contact with the skin, flesh (raw or not thoroughly
cooked), saliva, or other bodily fluids of domestic or
wild animals. Examples are rabies, plague, anthrax,
tularemia, and trichinosis
Horizontal disease transmission – from one individual to
another in the same generation (peers in the same age
group). Horizontal transmission can occur by either direct
contact (licking, touching, biting), or indirect contact air –
cough or sneeze
Vertical disease transmission – passing a disease causing
agent vertically from parent to offspring, such as perinatal
transmission
 DROPLET INFECTION
 Droplets or dust particles carry the pathogen to the
host and infect it
 Sneezing, coughing, talking all spray microscopic
droplets in the air
INDIRECT TRANSMISSION
pathogens or agents are transferred or carried by some
intermediate item or organism, means or process to a
susceptible host
Airborne Also known as the respiratory route, and the
resultant infection can be termed airborne disease. If
an infected person coughs or sneezes on another
person the microorganisms, suspended in warm, moist
droplets, may enter the body through the nose, mouth
or eye surfaces. Diseases that are commonly spread by
coughing or sneezing include:
 Chickenpox
 Common cold
 Influenza
 Mumps
Waterborne/vehicles borne
Transmission of communicable disease through water, food
,milk , blood or any other substances
Infection agent transmitted from reservoir to susceptible host
 Vector borne (3rd
organism)
 an organism called vector transmitt causative agent
of diseases from infected person to non infected
individual
 E.G mosquite,rat, lice, cockroach carry diseases like
malaria, yellow fever etc
FECAL-ORAL TRANSMISSION
 Direct contact is rare in direct route, for
humans at least. More common are the
indirect routes; foodstuffs or water become
contaminated (by people not washing their
hands before preparing food, or untreated
sewage being released into a drinking water
supply) and the people who eat and drink

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them become infected. This is the typical
mode of transmission for the infectious
agents of (at least):
 Cholera
 Hepatitis A
FOMITE BORNE:
Fomites are inanimate objects that can become
contaminated with infectious agents and serve as a
mechanism for transfer between hosts. The classic
example of a fomite is a park water fountain from
which many people drink. Infectious agents deposited
by one person can potentially be transmitted to a
subsequent drinker. However, many objects that we
come into contact with can serve as fomites;
doorknobs, elevator buttons, hand rails, phones,
writing implements, keyboards, toys in a day care
center, etc. Even a stethoscope can serve as a fomite if
it isn't cleansed.
METHODS OF PREVENTION OR CONTROL OF
COMMUNICABLE DISEASE
Communicable diseases:
 A disease which is transmitted from one
person to another directly or indirectly
through the infectious agent like food, air,
water, dust etc.
As discussed earlier that agent, mode of transmission and
host are very important for the spread of the disease if any of
these component is missing then disease cannot be spread.
Therefore measure should be taken to control these
components, so as to prevent the spread of disease.
1) controlling the source of infection:
The most desirable control measure would be to eliminate
the reservoir or source if that could be possible. Elimination
of the animal reservoir may be pretty easy i.e bovine, tb,
brucellosis but is not possible in humans.
1. Early diagnosis:
 The first step in the control of
communicable diseases its rapid
identification & accurate diagnosis of
disease
 e.g. measles, chicken pox
 if disease is properly treated
then the source and disease agent is
destroyed & the chances of the spread
of disease will be minimised.
Early diagnosis is needed for
a. The treatment of patients
b. For epidemiological investigations for example to
trace the source of infection from the known case to
the unknown or the primary source of infection
c. To study the time, place and person distribution(
descriptive epidemiology)
d. For the institution of prevention and control
measures
2.notification
Once a disease has been detected or even suspected, it should
be notified to the local health authority whose responsibility
is to put into operation control measures.
it is an important source of epidemiological information. It
enables early detection of disease outbreaks, which permits
immediate action to be taken by the health authority to control
their spread.
Notifications of infectious diseases is made by
a. Attending physician
b. Head of the family
Epidemiological investigations
An epidemiological investigation is called for whenever
there is disease outbreak.
These investigations covers the:
a. Identification of the source of infection
b. Factors influencing its spread in
community
These may include
a. Geographical situation
b. Climate condition
c. Social
d. Behavioral patterns
e. Character of the agent
f. Source
g. Vectors
h. Vehicles
i. Susceptible host population
4. Isolation:
it is an oldest communicable disease control measure.
It is defined as separation, for the period of communicability
of infected persons from others in such places & under such
conditions, as to prevent or limit the direct or indirect
transmission of infectious agent
Types:
There are several types of isolation which vary with the mode
of spread and severity of the disease
a. Standard isolation
b. Strict isolation
c. Protective isolation
d. High security isolation
Ways of isolation:

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a. In rural areas hospital isolation is better
than home isolation because it is
particularly difficult in these areas. As in
some situations such as cholera outbreaks
the entire village has to be isolated
b. Isolation can also be achieved by “ring
immunization” that is encircling the
infected persons with a barrier of immune
persons through whom the infection is
unable to spread. Eg. This method was
used worldwide to eradicate smallpox in
1960s or 1970s
Advantages:
a. Protection of community
b. Control of some infectious diseases eg.
Diphtheria, cholera
Disadvantages:
In some cases like polio, hepatitis and typhiod fever most
rigid isolation will not prevent the spread of the disease
It has failed in the control of diseases such as leprosy, tb and
stds
In these cases physical isolation has been replaced by
chemical isolation.
The duration of isolation is determined by the duration of
communicability of the disease and the effect of
chemotherapy on infectivity.
Examples:
 Chickenpox….duration of isolation: until all
lesions crusted; usually about days after
onset of rash
 Hepatitis: 3 weeks
 Influenza: 3 days after onset
 Polio: 2 weeks in adults, 3 weeks in pediatric
 Today isolation is recommended only when
the risk of transmission of the infection is
exceptionally serious.
Treatment:
Many communicable diseases have been tamed by
effective drugs.
The object of treatment is to kill the infectious agent when
it is still in the reservoir i.e before it is disseminated
.treatment reduces the:
1. Communicability of disease
2. Cuts short the duration of illness and
3. Prevents development of secondary cases
Types:
a. Individual treatment
b. Mass treatment
2) interruption of transmission:
A major aspect of communicable disease control relates to
“breaking” the chain of transmission.
E.g. water can be a medium for transmission of many
diseases as hepatitis a, dysentery, cholera so it should
be properly disinfected.
Human excreta should be disposed of in a sanitary way
Food borne diseases in areas having low standards of
sanitation so food should be protected.
 Overall standard of living should be
improved
 Vector- borne: control measures should be
directed primarily at the vector and its
breeding places. Mosquitos, flies, stray dogs
and other insects, rodents and stray dogs
should be destroyed.
All discharges of patients should be disposed off
 Food- borne: clean practices, hand washing,
adequate cooking, prompt refrigeration of
prepared food and withdrawal of
contaminated food
 Transmission of sexually transmitted
diseases can be prevented by using
mechanical contraceptives
3. The susceptible host:
The third link in the chain of transmission is the susceptible
host or people at risk. They may be protected by one or more
of the following strategies:
Immunity and immunization:
History:
Before polio vaccine became available in 1955, 58,000 cases
of polio occurred in peak years. ½ of these cases resulted in
permanent paralysis
Prior to measles vaccine in 1963, 4,000,000 cases per year
Immunization of 60 million children from 1963-1972 cost
$180 million, but saved $1.3 billion
Mumps used to be the leading cause of child deafness
10% of children with diphtheria died
◦ According to cdc, unless 80% or greater of the
population is vaccinated, epidemics can occur.
◦ Three types of immunity possible in humans:
Acquired immunity obtained by having had a dose of a
disease that stimulates the natural immune system or
artificially stimulating immune system
Active immunity body produces its own antibodies

15. 15
Can occur through a vaccine or in response to having a similar
disease Similar to acquire
Passive immunity (natural passive) acquired through trans
placental transfer of a mother’s immunity to diseases to the
unborn child (also via breastfeeding)
Can also come from the introduction of already produced
antibodies into a susceptible case
METHODS OF EPIDEMIOLOGICAL STUDIES
I] observational studies.
a) Descriptive studies
b) Analytical studies
-ecological
-case control
-cohort
Ii] Experimental studies.
a) Randomized control trial
b) Field trial
c) Community trial
Descriptive studies
 Is concerned with the study of frequency and
distribution of disease and health related events in
population.
Procedures
I defining the population to be studied.

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Ii defining the disease under study.
Iii describing the disease.
IV measurement of disease.
V comparing with known indices.
VI formulation of an etiological hypothesis
Analytical studies
 Analytical epidemiology focuses study on the
determinants of disease in a given population.
 analytical epidemiology is concerned with the
searching for the underlying causes
CASE CONTROL STUDY
 Can be called a retrospective study
 This type of study start from diagnosed cases.
 Here a group of people who have been diagnosed as
having a particular health problem (cases) are
compared with group of people who are free of that
particular problem (control).
 This method is called a retrospective study because
it is concerned with events in the past or investigator
look backward at the history of cause
 eg: cause of lung cancer is smoking, here the
investigator look backward at the history of smoking
in both the group.
 The case control method has three features.
1. Both exposure and outcome (disease) have
occurred before the starts of the study.
2. The study proceeds backward from effect
to cause.
3. It uses a control or comparison group to
support or refuse an inference.
Basic steps
I establish a case definition.
Ii identify sampling frame for selecting cases and controls.
Iii assess prior exposure status and other risk factors for cases
and controls.
IV compare prior exposure experiences of cases and controls
in order to estimate association between exposures and
health outcome.
Advantages
1. It is an excellent way to study rare diseases and diseases
with long latency,
2. A relatively quick answer can be obtained,
(3) It is relatively cheap,
(4) It usually requires only a few cases,
(5) It can often make use of existing records, and
(6) it can study several possible causes or
exposures to risk simultaneously
Disadvantages
1. It relies on subjects' recall and/or completeness
of existing records,
(2) It may be difficult or impossible to validate this
information, there is incomplete
allowance for extraneous factors,
(4) The selection of a suitable comparison (control)
group may be difficult, rates cannot be
calculated,
(6) The mechanism of disease cannot be studied,
and a proof of causation cannot be
established
COHORT STUDY
 Also known as
 prospective study
 forward looking study
 incidence study
A cohort is a specific group of people, at a certain
time, sharing common characteristics or experience
Eg: people born on the same day or the same year
This is usually under taken to obtain additional
evidence to support the suspected cause and
disease.
The distinguish features of cohort study are
1. The cohort are identified prior to the appearance
of the disease under investigation.
2.The study groups are observed over a period of
time to determine the frequency of disease.
3. The study proceeds forward from cause to effect.
1. In cohort study, the investigator select a group
of people for study and gather information
about who do and who do not have
characteristics in question.
2. eg: study about smoking and lung cancer,
those who smoke would be considered the
experimental group and those who do not
would be the control groups and assessing the
development of lung cancer in both the groups.

17. 17
3. The principle finding was that the experimental
group had an excess frequency of lung cancer
compare to control group.
4. since the disease had not yet occurred when the
study was under taken, this was a prospective
study.
Steps of cohort study
I selection of study subject.
Ii obtaining data on exposure.
Iii selection of comparison group.
IV follow up.
Vi analysis
Advantages
• – can determine incidence rates and
attributable risk directly.
• – exposure can be determined with
less potential for bias than if outcome
were already known.
• – can be efficient for studying rare
exposures.
DISADVANTAGES
• – inefficient, because must follow
many more subjects Than will
experience the event of interest.
• – Expensive.
• – Results not available for long time.
• – Bias due to attrition or loss to
follow-up.
EXPERIMENTAL STUDIES
 In experimental studies involve some
action, intervention or manipulation in the
experimental groups while making no
changes in the control groups, and
comparing the outcome of the experiment
in both groups.
Randomized controlled trial (rct)
 Is a form of human experimentation in
which the subjects, usually patients, are
randomly allocated to receive either a
standard accepted therapeutic or preventive
regimen, or an experimental regimen and
and analysis done
 Field trials
-Use people as unit of study.
 Community trial
- Use community as unit of study.
Epidemiological investigation
I verification of diagnosis
Ii confirmation of the existence of an epidemic
Iii defining the population at risk.
IV rapid search for all cases.
V data analysis
Vi formulation of hypothesis
Vii testing of hypothesis
Viii evaluation of ecological factors
Ix future investigation of population at risk.
X writing the report.
NATURAL HISTORY OF DISEASE
Definition
Natural history of disease refers to the progress of
a disease process in an individual over time, in the
absence of intervention.
The process begins with exposure to or
accumulation of factors capable of causing disease.
Without medical intervention, the process ends
with
– recovery ,
– disability,
– Or death.
–
Natural History of Disease

18. 18
Knowledge of the natural history of disease ranks
alongside causal understanding in importance
for disease prevention and control.
Natural history of disease is one of the major
elements of descriptive epidemiology
The natural history of disease is best established by
cohort studies.
As these studies are costly and laborious,
understanding of the natural history of disease is
largely based on other epidemiological studies,
such as cross-sectional and retrospective studies,
undertaken in different population settings.
• What the physician sees in the hospital is
just an "episode" in the natural history of
disease.
• The epidemiologist, by studying the natural
history of disease in the community setting
is in a unique position to fill the gaps in the
knowledge about the natural history of
disease.
• The natural history and spectrum of disease
presents challenges to the clinician and to
the public health worker.
• Because of the clinical spectrum, cases of
illness diagnosed by clinicians in the
community often represent only the “tip of
the iceberg.” Many additional cases may be
too early to diagnose or may remain
asymptomatic.
• For the public health worker, the challenge
is that persons with undiagnosed infections
may nevertheless be able to transmit them
to others.
Models of disease causation
• Germ theory of disease
• Epidemiological Triad
• Epidemiological Tetrad
• BEINGS theory
• Web of Causation theory
• Wheel theory
Germ Theory of Disease
• Proposed by Robert Koch and Louis
Pasteur.
• Every human disease is caused by a
microbe or germ, which is specific for
that disease and one must be able to
isolate the microbe from the diseased
human being.
Epidemiological Triad

19. 19
Agent
• s an element or substance, animate or
inanimate, the presence (or absence)
of which may initiate or perpetuate a
disease process.
• A disease may have a single agent, a
number of independent alternative
agents or complex of two or more
factors whose combined presence is
essential for the development of the
disease.
• Classification of agents:
• Biological
• Nutrient
• Physical
• Chemical
• Mechanical
• Absence or insufficiency or excess
of a factor necessary to health
• Social
Agent characteristics
• Infectivity refers to the proportion of
exposed persons who become
infected.
• Pathogenicity refers to the
proportion of infected persons who
develop clinical disease.
• Virulence refers to the proportion of
persons with clinical disease who
become severely ill or die.
• Hepatitis A virus in children has low
pathogenicity and low virulence, since
many infected children remain
asymptomatic and few develop severe
illness.
• In persons with good nutrition and health,
measles virus has high pathogenicity but
low virulence, since almost all infected
persons develop the characteristic rash and
illness but few develop the life-threatening
presentations of measles (pneumonia,
encephalitis).
• In persons with poor nutrition and health,
measles is a more virulent disease, with
mortality as high as 5-10%.
• Rabies virus is both highly pathogenic and
virulent, since virtually 100% of all
infected persons (who do not receive
treatment) progress to clinical disease and
death.
Host
• In epidemiological terminology, the
human host is referred to as “soil” and
the disease agent as “seed”.
• 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.
Environment

20. 20
• Physical environment
– Nonliving things and
physical factors(air,
water, soil, housing,
heat, light, etc)
• Biological environment
– Microbial agents,
insects, animals, plants
and man himself.
• Psychosocial environment
– Lifestyle, poverty,
urbanization,
community life,
income, education,
stress etc.
Epidemiological Tetrad
Disease Occurrence
Different diseases, in different communities, show
different patterns of expected occurrence:
Endemic: habitual presence of a disease within a
given geographic area.
Hyper endemic :a persistently high level of
occurrence.
Sporadic : an irregular pattern of occurrence, with
occasional cases occurring at irregular
Epidemic: occurrence in a community or region of
a group of illnesses of similar nature, clearly in
excess of normal expectancy and derived from a
common or from a propagated source.
• Public health officials often use the
term outbreak, which means the
same, because it is less provocative to
the public.
• When an epidemic spreads over
several countries or continents,
affecting a large number of people, it
is called a pandemic(worldwide
epidemic).
• intervals.(prevalence is zero)
The “BEINGS” Model of
Disease Causation
• This concept postulates that human
disease and its consequences are
caused by a complex interplay of nine
different factors –
Biological factors innate in a human being,
Behavioural factors concerned with individual
lifestyles,
Environmental factors as physical, chemical and
biological aspects of environment,
Immunological factors,
Nutritional factors,
Genetic factors,
Social factors,
Spiritual factors and
Services factors, related to the various aspects of
health care services
The Theory of “Web of Causation
• The “epidemiological triad theory”
was very effectively used by Leavel
and Clark in explaining the natural
history of disease and levels of
prevention for obviating such
departures from the state of health.
• But it could not explain the causation
of non communicable diseases like
IHD or road accidents.
• McMahon and Pugh forwarded the theory
of “epidemiological web of causation”,
wherein the various factors (e.g.
hypercholesterolemia, smoking,
hypertension) are like an interacting web of
a spider.
• Each factor has its own relative importance
in causing the final departure from the state

21. 21
of health, as well as interacts with others,
modifying the effect of each other.
Wheel theory
• As medical knowledge advanced, an
additional aspect of interest that came
into play is the comparative role of
“genetic” and the “environmental”
(i.e. extrinsic factors outside the host)
factors in causation of disease.
• The “triad” as well as the “web”
theory does not adequately cover up
this differential.
To explain such relative contribution of genetic
and environmental factors, the “wheel” theory has
been postulated
• The theory visualizes human disease
in the form of a wheel, which has a
central hub representing the genetic
components and the peripheral
portion representing the
environmental component.
• Like any wheel, the outer part
(environmental component) has
spokes (3 in this model) and the
environmental component is thus
divided into 3 sub components,
representing the social, biological and
physical components of the
environment.

22. 22
Factors of disease causation
• a) Predisposing factors are factors
which create a state of susceptibility,
so that the host becomes vulnerable to
the agent or to necessary cause, e.g.
age, sex, previous illness.
• b) Enabling factors are those which
assist in the development of (or in
recovery from) the disease; e.g.
housing conditions, socio-economic
status.
• c) Precipitating factors are those
which are associated with immediate
exposure to the disease agent or onset
of disease, e.g. drinking contaminated
water, close contact with a case of
pulmonary TB.
• d) Reinforcing factors are those which
aggravate an already existing disease, e.g.
malnutrition, repeated exposures
e) Risk factors: A risk factor is defined as a
condition, quality or attribute, the presence of
which increases the chances of an individual to
have, develop or be adversely affected by a disease
process. A risk factor is not necessarily the cause
of a disease but does increase the probability that a
person exposed to the factor may get the disease
Phase
1. Prepathogenesis phase
The disease agent has not yet entered man,
but the factors which favour its interaction
with the human host are already existing in
the environment.
2. Pathogenesis phase
The pathogenesis phase begins with the
entry of the disease “agent’’ in the susceptible
human host. The disease agent multiplies and
induces tissue and physiological changes, the
disease progresses through a period of incubation
and later through early and late pathogenesis
Factors
1.Agent factors
A substance, living or non-living, or a force,
tangible or intangible, the excessive presence or
relative lack of which may initiate or perpetuate a
disease process
 Biological agents:
These are living agents of disease, viruses,
rickettsiae, fungi, bacteria, protozoa
and metazoa.
These agents exhibit certain “host-related”
Biological properties such as:
 Infectivity:
This is the ability of an infectious agent to
invade and multiply in a host:
 Pathogenicity : This is the ability
to induce clinically apparent
illness..
 Virulence: the proportion of
clinical cases resulting in severe
clinical manifestations
 Nutrient agents:- These are proteins,
fats, carbohydrate, minerals and
water. Any excess or deficiency of the
intake of nutritive elements may
result in nutritional disorders. e.g.
Anaemia, goitre, obesity and vitamin
deficiencies are some of the current
nutritional problems in many
countries.
 Physical agents:-Exposure to
excessive heat, cold, humidity,
pressure, radiation, electricity, sound.
 Chemical agents :-

23. 23
o Endogenous: e. g. Ureamia, jaundice,
ketosis.
o Exogenous : e. g. allergens, metals, fumes.
 Mechanical agents:- Exposure of chronic
friction and other mechanical forces may
result in crushing, tearing, sprains,
dislocations and even death.
 Absence or insufficiency or excess of a
factor necessary to health
These may be:
(i) Chemical factors
(ii) Nutrient factors
(iii) chromosomal factors
(iv) Immunological factors.
 Social agents:- It is also necessary to
consider agents of disease. These are
poverty, smoking, abuse of drugs and
alcohol, unhealthy lifestyles, social
isolation, maternal deprivation
Host factors
The human host is referred to as “soil” and the
disease agent as “seed” . In some situations,
host factors play a major role in determining
the outcome of an individual’s exposure to
infection.eg. Tuberculosis.
The host factors may be classified as;
 Demographic
 Biological
 Social and economic characteristics such
as socioeconomic status, education,
occupation, stress, marital status , housing,
etc.
 Lifestyle factors such as personality traits
, living habits, nutrition, physical exercise,
use of alcohol, drugs and smoking,
behavioral patterns.
3 Environmental factors
 For human beings the environment is not
limited, as it normally is for plants and
animals, to a set of climatic factors.
 For Example, for man, social and
economic conditions are more important
than the mean annual temperature.
 Physical
 Biological
 Psychosocial.
 Physical environment:
The term “physical environment” is applied to
non-living things and physical factors (e.g.. Air,
water, soil, housing, climate, geography, heat,
light, noise, debris & radiation)
 Biological environment:- The
biological environment is the
universe of living things which
surrounds man, including man
himself. The living things are the
viruses and other microbial agents,
insects, rodents animals and plants
 Psychosocial environment:-
“Those factors affecting personal
health, health care and community
well-being that stem from the
psychosocial make-up of individuals
and the structure and functions of
social groups.”
Risk factor
a. An attribute or exposure that is significantly
associated with the development of a
disease.
b. A determinant that can be modified by
intervention, thereby reducing the possibility of
occurrence of disease or other specified outcomes.
Risk groups
Biological situation:
• age group, e.g., infants (low birth
weight), toddlers, elderly
• sex, e.g., females in the
reproductive age period

24. 24
• physiological state, e.g.,
pregnancy, cholesterol level, high
blood pressure
• genetic factors, e.g., family history
of genetic disorders
Physical situation:
• Rural, urban slums
• Living conditions , overcrowding
• Environment: water supply, proximity to
industries
c. Sociocultural and cultural situation:
• Social class
• Ethnic and cultural group
• Family disruption. Education, housing
• Customs, habits and behavior
• Lifestyles and attitudes
• Access to health services
SPECTRUM OF
DISEASE
The term “spectrum of disease” is a graphic
representation of variations in the
manifestations of disease.
Iceberg of disease
The floating tip of the iceberg represents what the
physician sees in the community, i.e., clinical
cases. The cast submerged portion of the
iceberg represents the hidden mass of disease,
i.e., latent, inapparent, presymptomatic and
undiagnosed cases and carriers in the
community. The “waterline” represents the
demarcation between apparent and
inapparent disease.
LEVEL OF PREVENTION
• Prevention is the action aimed at
eradicating, eliminating or
minimizing the impact of disease and
disability, or if none of these are
feasible, retarding the progress of the
disease and disability.
Primordial prevention
• Primordial prevention is defined as
prevention of risk factors themselves,
beginning with change in social and
environmental conditions in which
these factors are observed to develop,
and continuing for high risk children,
adolescents and young adults.
• It is the prevention of the emergence
or development of risk factors in
countries or population groups in
which they have not yet appeared.
• The main intervention in primordial
prevention is through individual and
mass education.
• Primordial prevention, a relatively
new concept, is receiving special
attention in the prevention of chronic
diseases. For example, many adult
health problems (e.g. obesity,
hypertension) have their early origins
in childhood, because this is the time
when lifestyles are formed(for
example, smoking, eating patterns,
physical exercise).

25. 25
• Primordial prevention begins in
childhood when health risk behaviour
begins. Parents, teachers and peer
groups are important in imparting
health education to children.
Examples:
• National policies and programes on
nutrition involving the agricultural sector,
the food industry, and the food import-
export sector
• Comprehensive policies to
discourage smoking
• Programes to promote regular
physical activity
• Making major changes in lifestyle
Primary prevention:
• Primary prevention can be defined as
the action taken prior to the onset of
disease, which removes the
possibility that the disease will ever
occur.
• It signifies intervention in the pre-
pathogenesis phase of a disease or
health problem.
• Primary prevention may be
accomplished by measures of “Health
promotion” and “specific protection”
• It includes the concept of "positive health",
a concept that encourages achievement and
maintenance of "an acceptable level of
health that will enable every individual to
lead a socially and economically productive
life".
• Primary prevention may be accomplished
by measures designed to promote general
health and well-being, and quality of life of
people or by specific protective measures.
Approaches for Primary Prevention
• The WHO has recommended the
following approaches for the primary
prevention of chronic diseases where
the risk factors are established:
– a. Population (mass)
strategy
– . High -risk strategy
Population (mass) strategy
• “Population strategy" is directed at
the whole population irrespective of
individual risk levels.
• For example, studies have shown that
even a small reduction in the average
blood pressure or serum cholesterol
of a population would produce a large
reduction in the incidence of
cardiovascular disease
• The population approach is directed
towards socio-economic, behavioral
and lifestyle changes
High -risk strategy
• The high -risk strategy aims to bring
preventive care to individuals at
special risk.
This requires detection of individuals at high risk
by the optimum use of clinical methods
Secondary prevention

26. 26
• It is defined as “ action which halts
the progress of a disease at its
incipient stage and prevents
complications.”
• The specific interventions are: early
diagnosis (e.g. screening tests, breast
self examination, pap smear test,
radiographic examinations, case
finding programme, etc) and adequate
treatment.
• Secondary prevention attempts to
arrest the disease process, restore
health by seeking out unrecognized
disease and treating it before
irreversible pathological changes take
place, and reverse communicability
of infectious diseases.
• It thus protects others from in the
community from acquiring the
infection and thus provide at once
secondary prevention for the infected
ones and primary prevention for their
potential contacts.
Early diagnosis and treatment
• WHO Expert Committee in 1973
defined early detection of health
disorders as “ the detection of
disturbances of homoeostatic and
compensatory mechanism while
biochemical, morphological and
functional changes are still
reversible.”
• The earlier the disease is diagnosed,
and treated the better it is for
prognosis of the case and in the
prevention of the occurrence of other
secondary cases.
Tertiary prevention
• It is used when the disease process
has advanced beyond its early stages.
• It is defined as “all the measures
available to reduce or limit
impairments and disabilities, and to
promote the patients’ adjustment to
irremediable conditions.”
• Intervention that should be
accomplished in the stage of tertiary
prevention are disability limitation,
and rehabilitation.
Impairment:
• Impairment is “any loss or
abnormality of psychological,
physiological or anatomical structure
or function.”
Disability:
• Disability is “any restriction or lack of
ability to perform an activity in the
manner or within the range
considered normal for the human
being.”
Handicap:
• Handicap is termed as “a
disadvantage for a given individual,
resulting from an impairment or
disability, that limits or prevents the
fulfillment of a role in the community
that is normal (depending on age, sex,
and social and cultural factors) for
that individual.”
Rehabilitation:
• Rehabilitation is “the combined and
coordinated use of medical, social,
educational, and vocational measures
for training and retraining the
individual to the highest possible
level of functional ability.”

27. 27
The combined and coordinated use of
medical, social, educational and vocational
measures for training and retraining the
individual to the highest possible level of
functional ability.”
The following areas of concern in rehabilitation
have been identified:
(a) Medical rehabilitation –
restoration of function
(b) Vocational rehabilitation –
restoration of the capacity to earn a
livelihood.
(c) Social rehabilitation – restoration
of family and social relationships
(d) Psychological rehabilitation –
restoration of personal dignity and
confidence.
IMMUNIZATION

28. 28

29. 29
PREVENTIO AND
CONTROL OF INFECTIOUS
DISEASE
Every disease has certain weak points
susceptible to attack. The basic approach
in controlling disease is to identify these
weak points and break the weakest links
in the chain of transmission.
Measures, pending results of epidemiologic
investigation:
 The reservoir or source of infection
 The route(s) of transmission
 The susceptible host (people at risk)
 . Controlling the reservoir
 If the first link in the chain of causation
(i.e.. the disease agent) is deemed to be the
weakest link, logically, the most desirable
control measure would be to eliminate the
reservoir or source, if that could be
possible. Elimination of the reservoir may
be pretty easy with the animal reservoir
(e.g.. bovine tuberculosis, brucellosis) but
is not possible in humans in whom the
general measures of reservoir control
comprise early diagnosis , notification,
isolation , treatment , quarantine ,
surveillance and disinfection - all
directed to reduce the quantity of the
agent available for dissemination.
 1) EARLY DIAGNOSIS
 The first step in the control of a
communicable disease is its rapid
identification
 ) NOTIFICATION

30. 30
 Once an infectious disease has been
detected (or even suspected), it should
be notified to the local health authority
whose responsibility is to put into operation
control measures, including the provision
of medical care to patients, perhaps in a
hospital.
 Certain diseases are statutorily notifiable
The diseases to be notified vary from
country to country: and even within the
same country Usually, diseases which are
considered 'to be serious menaces to public
health are included in the list of notifiable
diseases. Notifiable diseases may also
include non-communicable diseases and
conditions such as cancer congenital
defects, accidents, etc.
Notification is an important source of
epidemiological information. It enables early
detection of disease outbreaks, which permits
immediate action to be taken by the health
authority to control their spread. The other uses of
notification are discussed elsewhere.
Notification of infectious diseases is often
made by the attending physician or the head of the
family, but any one including the lay people (e.g..
Religious, political and administrative leaders,
teachers and others) can report, even on suspicion.
In all cases, the diagnosis is verified by the local
health authority
Under the International Health Regulations (IHR)
certain prescribed diseases are notified by the
national health authority to WHO. These can be
divided into:
 Those diseases subject to
International Health Regulations (1969).
Third Annotated Edition. 1983 cholera,
plague and yellow fever
 Diseases under surveillance by WHO -
louse-borne typhus fever, relapsing fever,
paralytic polio, malaria, viral influenza, etc
 Health administrations are required to
notify to WHO Geneva for any notification
of communicable diseases under
international surveillance and International
Health Regulations.
(3).EPIDEMIOLOGICAL INVESTIGATIONS
An epidemiological investigation is called for
whenever there is a disease outbreak the
methodology for which is given elsewhere.
Broadly, the investigation covers the identification
of the source of infection and of the factors
influencing its spread in the community. These may
include geographical situation, climatic condition,
social, cultural and behavioral patterns, and more
importantly the character of the agent, reservoir the
vectors and vehicles, and the susceptible host
populations.
4).ISOLATION
Isolation is the oldest communicable disease
control measure. It is defined as "separation, for the
period of communicability, of infected persons or
animals from others in such places and under such
conditions, as to prevent or limit the direct or
indirect transmission of the infectious agent from
those infected to those who are susceptible, or who
may spread the agent to others". In general,
infections from human/animal sources can be
controlled by physical isolation of the case or
carrier and it necessary, treatment until free from
infection, provided cases and carriers can be easily
identified and carrier rates are low.
The purpose of isolation is to protect the
community by preventing transfer of infection
from the reservoir to the possible susceptible hosts.
The type of isolation varies with the mode of spread
and severity of the disease. There are several types
of isolation - standard isolation, strict isolation,
protective isolation, high security isolation. For
each patient, the relative risks to the patient and to
others should be assessed and the appropriate type
of isolation determined. Hospital isolation,
wherever possible, is better than home isolation.
Isolation is particularly difficult in rural areas. In
some situations (e.g., cholera outbreaks) the entire
village or rural community may have to be isolated.

31. 31
Isolation may also be achieved in some diseases by
"ring immunization” that is encircling the infected
persons with a barrier of immune persons through
whom the infection is unable to spread. This
method when applied worldwide in the 1960s and
1970s eradicated smallpox. In North America, ring
immunization is being applied in measles control
and eradication. The duration of isolation is
determined by the duration of communicability of
the disease and the effect of chemotherapy on
infectivity.
Isolation has a distinctive value in the control
of some infectious diseases , e. G. , diphtheria
, cholera, streptococcal respiratory disease,
pneumonic plague, etc. In some diseases where
there is a large component of subclinical infection
and carrier state (polio, hepatitis A, and typhoid
fever), even the most rigid isolation will not
prevent the spread of the disease. It is also futile to
impose isolation if the disease is highly infectious
before it is diagnosed as in the case of mumps.
Isolation has tailed in the control of diseases such
as leprosy, tuberculosis. In the control of these
diseases, the concept of physical isolation has been
replaced by chemical isolation, i.e., rapid treatment
of cases in their own homes and rendering them
non-infectious as quickly as possible. Lastly,
cases are usually reported after the disease has
spread widely.
Taking all these limitations into consideration, it
may be stated that isolation which is a "barrier
approach" to the prevention and control of
infectious disease is not as successful as one would
imagine and may well give rise to a false sense of
security . In modern-day disease control, isolation
is more judiciously applied and in most cases
replaced by surveillance because of improvements
in epidemiological and disease control
technologies. Today isolation is recommended
only when the risk of transmission of the infection
is exceptionally serious.
5) TREATMENT
Many communicable diseases have been tamed by
effective drugs. The object of treatment is to kill the
infectious agent when it is still in the reservoir, i.e...
Before it is disseminated. Treatment reduces the
communicability of disease, cuts short the duration
of illness and prevents development of secondary
cases. In some diseases (e.g., syphilis, tuberculosis
and leprosy), early diagnosis and treatment is of
primary importance in interrupting transmission.
Treatment is also extended to carriers.
Treatment can take the form of individual treatment
or mass treatment. In the latter category, all the
people in the community are administered the
drugs whether they have the disease or not (e.g..
Trachoma). If the treatment is inadequate or
inappropriate, it may induce drug resistance in the
infectious agent and may frustrate attempts to
control the disease by chemotherapy It is well to
remember that no disease has ever been conquered
through attempting to treat every affected
individual.
6) QUARANTINE
Quarantine has been defined as "the limitation of
freedom of movement of such well persons or
domestic animals exposed to communicable

32. 32
disease for a period of time not longer than the
longest usual incubation period of the disease, in
such manner as to prevent effective contact with
those not so exposed“. Quarantine measures are
also "applied by a health authority to a ship, an
aircraft, a train, road vehicle, and other means of
transport or container, to prevent the spread of
disease, reservoirs of disease or vectors of disease
Quarantine may comprise:
 Absolute quarantine
 Modified quarantine, e.g.. A selective
partial limitation of freedom or movement,
such as exclusion of children from school
 Segregation which has been defined as "the
separation for special consideration, control
of observation of some part of a group of
persons (or domestic • animals) from the
others to facilitate control of a
communicable disease, e.g.. Removal of
susceptible children to homes of
immune persons"
 In contrast to isolation, quarantine applies
to restrictions on the healthy contacts of an
infectious disease. Quarantine which was
once a popular method of disease control
has now declined in popularity. With better
techniques of early diagnosis and treatment,
quarantine, as a method of disease control,
has become outdated. It has been replaced
by active surveillance.
II. Interruption of transmission
A major aspect of communicable disease control
relates to “breaking” the chain of
transmission. This may mean changing
some components of man’s environment to
prevent the infective agent from a patient or
carrier from entering the body of
susceptible host. For example, water can be
a medium for transmission of many disease
such as dysentery, hepatitis A, cholera.
Water treatment will eliminated these
diseases. Depending upon the level of
population, this may vary from simple
chlorination to complex treatment.
 Food–borne disease is particularly
prevalent in areas having low standards of
sanitation. Clean practices such as hand
washing, adequate cooking, prompt
refrigeration of prepared foods and
withdrawal of contaminated food will
prevent most When the disease is vector-
born, control measures should be directed
primarily at the vector and its breeding
places.
 Food-borne illnesses.
 On the other hand, episodes of
infection either by droplets or
droplet nuclei are not usually
controlled effectively by attempting
to interrupt their mode of spread;
reliance is placed on early diagnosis
and treatment of patients, personal
hygiene and proper handling of
secretions and excretions.
III. The susceptible host
The third link in the chain of transmission is the
susceptible host or people at risk. They my
be protect by:
 Active immunization
Active immunization is one of the most powerful
and cost-effective weapons of the modern
medicine.
Active immunization
Passive immunization
Passive immunization is a short-term expedient
useful only when exposure to infection has just
occurred or is imminent within the next few days.
The duration of immunity induced is short and
variable-1-6 weeks.
 Combined active and passive
immunization
Chemoprophylaxis
Non-specific measures

33. 33
 Improvement in the quality of life
(e.g. Better housing, water supply,
sanitation, nutrition, education) fall
into this category.
DISINFECTION
 Disinfection: the inactivation of
disease-producing microorganisms.
Disinfection does not destroy
bacterial spores. Disinfectants are
used on inanimate objects in contrast
to antiseptics, which are used on
living tissue. Disinfection usually
involves chemicals, heat or ultraviolet
light. The nature of chemical
disinfection varies with the type of
product
 High level disinfection High level
disinfection processes destroy vegetative
bacteria, mycobacteria, fungi and
enveloped (lipid) and no enveloped (non
lipid) viruses, but not necessarily bacterial
spores. High level disinfectant chemicals
(also called chemical sterilants) must be
capable of sterilization when contact time is
extended. Items must be thoroughly
cleaned prior to high level disinfection.
Intermediate level disinfection
Intermediate level disinfectants kill
vegetative bacteria, most viruses and
most fungi but not resistant bacterial
spores
 Low level disinfection Low level
disinfectants kill most vegetative bacteria
and some fungi as well as enveloped (lipid)
viruses (e.g., hepatitis B, C, Hantavirus, and
HIV). Low level disinfectants do not kill
mycobacteria or bacterial spores. Low level
disinfectants are typically used to clean
environmental surfaces.
 Microorganisms. Disinfection Chemical
Methods does not destroy bacterial spores.
Chemical method
 Disinfectants and antiseptics
 Surface-active agents
(surfactants)
 Chemical food preservatives
 Aldehydes
 Gas sterilization