EPIDEMIOLOGY

UNIT-III
EPIDEMIOLOGY
R. SRIDEVI
HOD , CHN DPT
GCON, RIMS

introduction
• The term epidemiology is derived from the Greek word epidemic.
• – Epi means-Among, upon,
• – Demos means study population or people and
• – Logos means scientific study.
• So
• – it is the scientific study of the disease pattern in human
population.
• – In broad sense, it is the study of effects of multiple factors on
human health.
• – It is multidisciplinary subject involving those of the physician,
Biologists, Public Health experts, Health educators etc.

Definitions
• Epidemiology as, study of the distribution and
determinants of diseases frequency in man.
(Mac Mohan and Pugh)
• The study of the disease, any diseases, as a mass
phenomenon. (Greenwood 1935)
• "The study of the distribution and determinants of
health related states or events in specified population
and the application of the study to of health problems“
(J.M. Last 1988)

Objectives of epidemiology
• Study of frequency and distribution of health and health
related problems in community at large.
• Identification of determinants i.e. etiological factors
causing health and health related problems.
• Need based planning and administration of
comprehensive health care programmes with the available
resources to deal with health and health related problems
• • Evaluating the effectiveness of the programmes to
provide feedback.

USES OF EPIDEMIOLOGY
1. To study the rise and fall of disease in the population
2. community diagnosis
3. Planning and evaluation
4. Evaluation of Individual's risk and chances
5. Syndrome identification
6. Completing the natural history of disease
7. Searching for cause and risk factors.

1. Determination of the origin of a disease whose cause is known
2. Investigation and control of a disease whose cause is either
unknown or poorly understood.
3. Acquisition of information on the ecology and natural history of a
disease.
4.Planning and monitoring of disease control programmes.
5. Assessment of the economic effects of a disease and analysis of
the costs and economics
benefits of alternative control programmes.

Determine the usefulness and effectiveness of
new/innovative techniques, measures and programmes
 Complete the clinical picture of chronic diseases & slow
growing diseases
Identify syndromes by describing the distribution and
association of clinical phenomena in the population.
 Forecast the likely occurrence of diseases on the basis
of epidemiological principles

purposes
• To prevent, control and eradicate health and
• health related problems.
• To reduce/minimize the impact of the problems.
• To promote health and quality of life of people
at large.

Terminology
• Endemic
When an infectious disease more or less
prevailing on a locality or community called as
endemic • E.g.. Chickenpox
• Pandemic
When an epidemic spread from one country to
another or even whole world infecting most of
the population then the conditions called as
pandemic• E.g.. Swine flu

Epidemic
• Sudden out break of Infectious disease that spreads rapidly
through Population affecting a large number of population in short
period of time is called as epidemic • E.g.. AIDS in Africa
DISEASE:
A pattern of response by a living organism to some form of
invasion by a foreign substance or injury which causes an
alteration of the organisms normal functioning also – an abnormal
state in which the body is not capable of responding to or carrying
on its normally required functions
PATHOGENS:
organisms or substances such as bacteria, viruses, or parasites that
are capable of producing diseases
PATHOGENISES:
the development, production, or process of generating a disease

PATHOGENIC:
means disease causing or producing agent
PATHOGENICITY:
describes the potential ability and strength of a pathogenic
substance to cause disease
INFECTIVE:
diseases are those which the pathogen or agent has the capability to
enter, survive, and multiply in the host
VIRULENCE:
The ability of an agent of infection to produce disease. The virulence
of a microorganism is a measure of the severity of the disease it
causes.
INVASSIVNESS:
the ability of microorganisms to enter the body and spread in the
tissues.
2. the ability to infiltrate and actively destroy surrounding tissue,
a property of malignant tumors.

HYPER ENDEMIC:
Diseases that affect a high proportion of population at risk.
HOLO ENDEMIC:
Disease that is highly prevalent in a population & is commonly
acquired early in life, in most all of the children of the population
MESO ENDEMIC:
Diseases that affect a moderate proportion of population at risk.
HYPO ENDEMIC:
Diseases that affect a small proportion of population at risk.
SPORADIC:
sporadic is something that happens occasionally or at irregular
intervals.
INCIDENCE: ( NEW CASES)
measure of the probability of occurrence of a given medical condition in a
population within a specified period of time. Although sometimes loosely
expressed simply as the number of new cases during some time period, it is better
expressed as a proportion or a rate with a denominator.

PREVALENCE: ( old +new)
The number of people within a population who have a certain disease at a
given point in time
POINT PREVALENCE:
How many cases of a disease exist in a group of people at that moment.
AGENT: is the cause of the disease Can be bacteria, virus, parasite, fungus,
mould Chemicals (solvents), Radiation, heat, natural toxins (snake or spider
venom)
HOST:
is an organism, usually human or animal, that harbours the disease
ENVIRONMENT:
is the favourable surroundings and conditions external to the human or
animal that cause or allow the disease or allow disease transmission

VECTOR:
Any living non-human carrier of disease that transports and serves
the process of disease transmission Insects: fly, flea, mosquito;
rodents; deer
RESERVOIRS:
humans, animals, plants, soils or inanimate organic matter (feces or
food) in which infectious organisms live and multiply Humans often
serve as reservoir and host
ZOONOSIS:
When a animal transmits a disease to a human
INFECTION:
The entry & development or multiplication of disease producing
agent in or on body of man/animal Is called infection.
INCUBATION PERIOD:
Time interval b/w the entry of diseased agent into the body of host
appearances of first sign & symptoms of disease.

INFECTIOUS AGENT:
Any agent which is capable of producing an infection is called
infectious agent.
INFESTATION:
An infestation is the presence of animal parasite either externally
or internally.
CONTACT:
Any person who has remain in association with the infected
person or the infected particles can also develop the disease.
CONTAGIOUS DISEASE:
A disease which is transmitted by contact.
LATENT PERIOD:
The period between exposure and the onset of the period of
communicability, which may be shorter or longer than incubation
period.

Dynamics of disease transmission
1. The Reservoir.
2. Mode of Transmission.
3. Susceptible Host.

RESERVOIR
• A reservoir is defined as “any person, animal, arthropod,
plant, soil or substance in which an infectious agent
lives and multiplies , on which it depends primarily for
survival, and where it reproduces itself in such manner that
it can be transmitted to a susceptible host”.

Reservoir of infectious agent
Eg:-
In hookworm infection, the reservoir is man and the
source of infection is soil contaminated with infective
larvae.
In typhoid fever the reservoir is a case or carrier is man
and the source of infection is faeces or urine of patient or
contaminated food and water.
Types of reservoirs:
1. Human reservoir
2. Animal reservoir
3. Reservoir in non living things

Human reservoir
The most important source or reservoir
of infection for human is man himself.
Human may be
Case
Carrier

Human reservoirs are 2 types
• 1. Cases : A case is defined as “ a person in the
population having the particular disease, health
disorder or condition under investigation”.
The cases are of following types:
Clinical cases
Sub clinical cases
Latent cases

1. clinical illness:-
• Clinical illness may be mild or moderate, typical or atypical, severe
or fatal.
• Mild cases may be more important source of infection than severe
cases.
ii. Sub clinical cases
• Sub clinical cases are also known as in apparent, missed or
abortive cases.
• The disease agent may multiply in the host but does not manifest
itself by signs and symptoms.
• Sub clinical infection may be detected only by laboratory tests.
iii. Latent infection
• When a virus is present in the body but exists in a resting (latent)
state without producing more virus. A latent viral infection usually
does not cause any noticeable symptoms and can last a long period
of time before becoming active and causing symptoms.

2. CARRIERS
• In some diseases, either due to inadequate treatment
or immune response, the disease agent is not completely
eliminated, leading to a carrier state.
• A carrier is defined as an infected person or animal that
harbours a specific infectious agent and serves as a
potential source of infection for others.
The elements in a carrier state are:-
The presence of disease agent in the body.
The absence of recognizable signs and symptoms
Spread of disease agent in the discharges or excretions.

A. TYPE
• (a) Incubatory carriers:
• Carriers which spread the infectious agent during the incubation
period of disease.
• This usually occurs during the last few days of incubation period.
• Eg:- measles, mumps, polio, influenza, hepatitis B
(b) Convalescent carriers:-
• Carriers which continue to spread disease during the period of
convalescence.
• Eg ;-Typhoid, Dysentery, Cholera, Diptheria
(c) Healthy carriers:-
An asymptomatic carrier (healthy carrier or just carrier) is a person
or other organism that has become infected with a pathogen, but
that displays no signs or symptoms. Although unaffected by the
pathogen, carriers can transmit it to others or develop symptoms in
later stages of the disease.

B. DURATION
• (a) Temporary carriers:-
• Carriers which spread infectious agent for short period of
time
• (b) Chronic carriers:-
• Carriers which spread infectious agent for indefinite
period
C. PORTAL OF EXIT
Urinary
Intestinal
Respiratory
Others –(skin eruptions, open wounds,blood)

2. ANIMAL RESERVOIR
• The source of infection may sometimes be animals and
birds.
• The diseases and infections which are transmissible to
man from vertebrate are called zoonoses.
• Eg :- Rabies, Yellow Fever, Influenza
3. RESERVOIR IN NON LIVING THINGS:
• Soil and inanimate matter can also act
as reservoir of infection

Mode of trasmission
• DIRECT TRANSMISSION
• • Direct contact
• • Droplet infection
• • Contact with soil
• • Inoculation into skin or mucosa
• • Trans placental or vertical transmission

INDIRECT TRANSMISSION
• Vehicle borne
• Vector borne
• Air borne
• Fomite borne
• Unclean hands & fingers
Indirect Transmission
5 ‘F’
food, flies, fomite, finger, fluid

1. Direct contact
• Infection may be transmitted by direct contact from
skin to skin, mucosa to mucosa, or mucosa to skin.
• Eg :- STD, AIDS, leprosy, leptospirosis, skin and eye
infections
2. Droplet infection
• This is direct projection of spray of droplets of saliva
and nasopharyngeal secretions during coughing,
sneezing, talking or spitting.
• The droplet spread is usually limited to a distance of 30-
60 cm between source and host
• Eg :-Respiratory Infections, Common Cold, Tuberculosis,
Diphtheria.

3. Contact with soil:-
• The disease agent may be acquired by direct exposure to the
disease agent in the soil
• Eg :- hook worm, tetanus, mycosis
• 4. Inoculation into skin:-
• Disease agent may be inoculated directly into the skin or mucosa
• Eg:-rabies virus by dog bite, Hepatitis B by contaminated needles
5. Transplacental transmissin:-
• Disease agents can be transmitted transplacentally.
– S-Syphilis
– T-Toxoplasma
– O-Other infections(AIDS, varicella, Hepatitis B)
– R-Rubella virus
– C-Cytomegalo virus
– H-Herpes virus

Indirect transmission
1. VEHICLE BORNE
• Vehicle borne transmission implies transmission of the infectious
agent through the agency of water, food, raw vegetables, fruits, milk,
blood etc
.
Classification of vector borne disease:-
• 1. By vector
– Invertebrate Eg-arthropods( flies, mosquitoes, cockroach, ticks,
mites, bugs)
– Vertebrate Eg-mice, rodents
2. By transmission chain
• Man and a non vertebrate host (man-mosquitoe- man in malaria)
• Man , another vertebrate host and a non vertebrate host (bird-
arthropod-man)
• Man and 2 intermediate host (man-cyclops-fish-man)

FOMITE BORNE
• Fomites are inanimate articles or substances other
than water or food contaminated by infectious agents.
• Eg- soiled clothes, syringes, instruments etc.
5. UNCLEAN HANDS
• Lack of personal hygiene favour personperson
transmission of infection.

SUSCEPTIBLE HOST:
FOUR STAGES:
PORTAL OF ENTRY
SITE OF ELECTION
PORTAL OF EXIT
LOW GRADE IMMUNITY

DISEASE CYCLE:
INCUBATION PERIOD
PRODROMAL PERIOD
FASTIGIUM
DEFERVESCENCE
CONVALESCENCE
DEFECTION

INTUBATION PERIOD: the period between exposure to an infection and the
appearance of the first symptoms of disease
Prodromal period:
This period is of short duration, which ranges from one to four days.The pathogenic
agent multiply in human host and the vague system appear .
Fastigium:
The symptoms are clear cut and are related to particular disease.
Disease can be identified by clinical diagnosis. the most severe point in the course
of an illness
DEFERVESCENCE The body's defence begin to respond and there is decline of
infection. The clients condition starts improving and feels better.
CONVALESCENCE: Convalescence is the gradual recovery of health and strength
after illness or injury. It refers to the later stage of an infectious disease or illness
DEFECTION:
The patient's condition is improved and comes to his pre illness
stage i.e is free from illness as the recovery has occured.

THEORIES AND MODELS OF DISEASE
CAUSATION
1. Germ theory
2. Epidemiological traid theory
3. Multifactorial causation theory
4. Web causation theory

It states that microorganisms known as pathogens or "germs"
can lead to disease. These small organisms, too small to see
without magnification, invade humans, other animals, and other
living hosts. Their growth and reproduction within their hosts
can cause disease
"Germ" may refer to not just a bacterium but to any type of
microorganism or even non-living pathogen that can cause
disease, such as , fungi, viruses, prions (misfolded proteins),
or viroids ( plant pathogens) Diseases caused by pathogens are
called infectious diseases.

EPIDEMILOGICAL TRAID MODEL
• The germ theory of disease has many limitations
• This model showing the interaction and interdependence of agent , host,
environment

The Analytical Epidemiologic Triad
This model comprises a susceptible host (the person at risk for
the disease), a disease agent (the proximate cause), and an
environmental context for the interaction between host and
agent.
Thus, development of disease is a combination of events:
• A harmful agent
• A susceptible host
• An appropriate environment
September 8, 2014 Epidemiological Triads 45

Agents
• Biological (micro-organisms)
• Physical (temperature, radiation, trauma, others)
• Chemical (acids, alkalis, poisons, tobacco,
medications / drugs, others)
• Environmental (nutrients in diet, allergens, others)
• Nutritional (under- or over-nutrition)
• Psychological experiences

Host Factors
• Host factors are intrinsic factors that influence an
individual’s exposure, susceptibility, or response
to a causative agent. These include:
• Genetic endowment
• Immunologic state
• Personal behavior (life-style factors): diet,
tobacco use, exercise, etc
• Personal characteristics (described before, under
“person”), including: age, gender, socio-economic
status, etc.

Environmental factors are extrinsic factors which affect the agent
and the opportunity for exposure. These include:
– Physical factors: e.g. geology, climate (temperature,
humidity, rain, etc)
– Biological factors: e.g. insects that transmit an agent
– Socioeconomic factors: e.g. crowding, sanitation, and the
availability of health services
Phenomena which bring the host and agent together: vector,
vehicle, reservoir, etc
Environment

•Agent factors include infectious microorganisms, e.g. virus,
bacterium, parasite, or other agents.
•They may be necessary but not always sufficient alone to cause
disease.
•Host factors are intrinsic factors that influence an individual’s
exposure, susceptibility, or response to a causative agent
•Environmental factors are extrinsic factors which affect the agent and
the opportunity for exposure.
Summary of Analytical Triad

WEB OF CAUSATION
According to this disease never depends upon single
isolated cause rather it develops from a chain of
causation in which each link itself is a result of
complex interaction of preceding events these
chain of causation which may be the fraction of
the whole complex is known as web of causation.

ICEBERG OF DISEASE
• Disease in a community is compared to an iceberg.

The floating tip of ice berg represents clinical
cases
Submerge portion shows undiagnosed cases
or hidden mass of disease
Undiagnosed reservoir of disease is a
challenge to modern techniques in preventive
medicine

CONCEPTS OF DISEASE CONTROL
The term disease control refers ongoing operation aimed at reducing:
o The incidence of disease.
o The duration of disease and the consequently the risk of transmission.
o The effect of infection including physical and psychological
complication.
o The financial burden to the community.
DISEASE MONITORING:
• Defined as “the performance and analysis of routine measurement aimed
at detecting changes in the environment or health status of population.”
e.g. growth monitoring of child, Monitoring of air pollution, monitoring of
water quality etc.
• DISEASE SURVEILLANCE:
• Defined as “the continuous scrutiny of the factors that determine the
occurrence and distribution of disease and other conditions of ill health.”
E.g. Poliomyelitis surveillance programme of WHO.

CONCEPTS OF PREVENTION
The goals of medicine are to
• Promote health,
• To preserve health,
• To restore health when it is
impaired
• And to minimize suffering and
distress.
These goals are embodied in the word
"prevention"

Actions 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.
• The concept of prevention is best defined in the
context of levels, traditionally called primary, secondary
and tertiary prevention. A fourth Level, called primordial
prevention, was later added.

1. PRIMORDIAL PREVENTION
• DEFINITION
“It is the prevention of the emergence or development of risk
factors in countries or population groups in which they have not yet
appeared.”
• INTERVENTION
The main intervention in primordial prevention is
through individual and mass health education.

Health promotion
 Health education
Environmental modifications
Nutritional interventions
 Lifestyle and behavioural changes.
Health education to improve healthy habits and health consciousness in the
community.
 Improvement in nutritional standards of the community.
 Healthful physical environment (Housing, water supply, excreta disposal, etc.,)
 Good working condition
Marriage Counselling
Periodic Selective examination of risk population.

Specific protection
 Use of Specific immunization (BCG, DPT,MMR vaccines)
 Chemoprophylaxis (tetracycline for Cholera, dapsone for
Leprosy, Chloroquine for malaria,etc.,)
 Use of specific nutrients (vitamin A for Children, ironfolic acid
tablets for Pregnant mothers)
 Protection against accidents (Use of helmet, seatbelt,etc.,)
Protection against occupational hazards.
Avoidance of allergens.
 Protection from air pollution.

SECONDARY PREVENTION
Definition: The action which halts the progress of a disease
at its incipient stage and prevents complications”.
INTERVENTIONS:
Individual and mass case-finding measures.
 Screening surveys(urine examination for diabetes,etc.,)
Selective examination

TERTIARY PREVENTION
Tertiary prevention can be defined as all measures
available to reduce or limit impairments and
disabilities, minimize suffering caused by existing
departures from good health and to promote the
patient adjustment to irritable conditions
MODES OF INTERVENTION:
1. Disability limitation
2. Rehabilitation

REHABILITATION:
Medical rehabilitation: (restoration o Bodily Function).
Vocational rehabilitation:( restoration of the capacity to earn a
livelihood)
Social rehabilitation: (restoration of family and social
relationship).
Psychological rehabilitation: (Restoration of personal dignity and
confidence)
EXAMPLES FOR REHABILITATION:
 Establishing schools for the blind.
Prevention of aids for the crippled.
 Reconstructive surgery in Leprosy.
 Change of profession for a more suitable one and modification of
life in general in the case of TB, etc.,

mortality & morbidity
Measurements

1. RATE
2. RATIO
3. PRAPORTION

A fraction is made up of 2 numbers.
The top number is called the
NUMERATOR
and the bottom number is called the
DENOMINATOR.
In the fraction ¾ the 3 is the numerator
and the 4 is the denominator.

No of death in one year
Death rate= - - - - - - - - - - - - - - - - - - - - X 1000
• Total mid year population
•Numerator
Denominator.
Time
specification
Multiplier
• (Numerator is part of denominator )

The value obtained by dividing one
quantity by another- X/Y.
Male to female ratio.
A ratio often compares two rates,
death rates for women and men at a
given age.

Ratio also expresses relation of size
between the two quantities.
Numerator is not part of Denominator.
Expressed as X / Y.
Doctor : Population ratio.
Male : Female ratio.
WBC: RBCratio

A part/share or number considered in
comparative relation to a whole.
Usually expressed as a percentage %

This is also relation /magnitude between
two quantities, And numerator is always
part of denominator.
And expressed as percentage
-Proportion of female students .
-Proportion of anemic mothers
(60% mothers are anemic)

Incidence
Occurrence of new cases
•
Prevalence
Existence of all new & old cases.

Prevalence:- how many people in
a population currently have the
disease.
Incidence:- how many people are
diagnosed each year

• The rate at which acute
disease is spreading - -
used during epidemics
& expressed in %.
Attack rate
• %of exposed persons
developing disease after
primary case exposure
Secondary
attack rate

• Prevalence at any
given point of time.
• 4%TB cases on 1st April
Point
prevalence
• Prevalence at a given
period of time.
• Period will be 1year.
Period
prevalence

Longer duration of the disease.
Prolongation of life, with treatment.
If incidence increases.
Immigration of new cases.
Better reporting of cases.
Emigration of healthy people.

Longer
duration of
disease
Incidence
increases.
Prolongation
of life
without
cure.
20

Shorter duration of diseases.
Improved cure rate.
Incidence decreases.
Emigration of new cases.
Under reporting of cases.
Immigration of healthy
people.

Improved cure rate.
Short duration of
disease.
Incidence decreases

Crude Death Rate.
Specific death rate.
Case fatality rate.
Proportional mortality rate.
Survival rate.
Standardized death rate.

Number of deaths from all causes, per
1000 estimated mid year population in
one year in a given place.
No deaths during one year
CDR = X 1000
Mid year population

Cause Specific death rate like
disease death rate, Road accident…
Age specific-IMR, Child Mortality rate
Sex specific death rate – MMR/female
Period specific death rate–Death in May

Percentage of particular cases dying
during particular disease epidemic.
Killing power of disease
particularly acute diseases
No of deaths due to cholera
CFR= - - - - - - - - - - - - - - - - - - - - - - - X 100
Total No of cholera cases

Proportion or %of deaths due to
particular cause out of total deaths.
It measures the disease burden.
Under 5,
proportional =
mortality rate
No of deaths below 5 years
- - - - - - - - - - - - - - - - - - - - X 100
Total No all of deaths

Percentage of the treated patients remaining
alive at the end of 5 years treatment.
Yard stick for assessing the standard of
therapy in cancer.
Survival
Rate
pts alive at the end of 5 yrs
= - - - - - - - - - - - - - - - - - - - - - - X 100
Total No of pts treated

CDR can not be useful for
comparison.
Death rate need to be standardized
for comparisons.
Standardization can be done by-
:adjusting death rate age wise,
:also can be done sex/race wise

EPIDEMIOLOGICAL METHODS
1. OBSERVATIONAL STUDIES
 Descriptive studies
 Analytical studies
2. EXPERIMENTAL STUDIES
Randomized control trials
 Field trials
 Community trials

OBSERVATIONAL STUDIES
Do not have control over the circumstances Allow nature
to take its own course, the investigator measures but
does not intervene
1.Descriptive Epidemiology:
Descriptive epidemiology is the study of amount and
distribution of disease or health status with in a population by
person, place and time.
When is the disease occurring?–time distribution.
Where is it occurring? –place distribution.
Who is getting the disease?-person.

USES OF DESCRIPTIVE EPIDEMIOLOGY
Provide data regarding the magnitude of the disease
load & types of disease problems in the community in
terms of morbidity & mortality rates & ratios.
Provide clues to disease aetiology & help in the
formulation of an etiological hypothesis.
Provide back ground data for planning, organizing , &
evaluating preventive & curative services.
Contribute to research by describing variation in
disease occurrence.

Procedures in descriptive studies
1. Define the population to be studied
2. Defines the disease
3. Describe the disease
4. Time, place and person
5. Measurement of disease
6. Comparing with known indices
7. Formulation of hypothesis.

1. Defining the population to be studied
1.Descriptive studies are investigations of
populations not individuals
.The defined population can be:
o The whole population
o A representative sample

2. DEFINING THE DISEASE UNDER STUDY
The epidemiologist looks out for an "operational
definition", i.e..a definition by which the disease or
condition can be identified and measured in the
defined population with a degree of accuracy

3.DESCRIBING THE DISEASE
Describes the occurence and distribution of disease by time,
place and person and identifying those characteristics
associated with presence or absence of the disease in
individuals
TIME PLACE PERSON
Year, season Climatic
zones
age Birth order
Month, week Country,
region
sex Family size
Day, hour of
onset
Urban/rural Marrital
state
Height,weigh
t
duration Towns, cities Occupation,
social status,
education
BP, blood
cholestrol,
personal habbits

4.PLACE DISTRIBUTION:
• International variations:(cancer)
• National variations:(distribution of goitre, fluorosis,
leprosy, malaria etc)
• Rural –Urban variations(chronic bronchitis, accidents,
mental illness in urban areas, soil transmitted
helmenthies, skin, zoonotics diseases)
PERSON DISTRIBUTION:
• Age; measels-child hood
• Middle age-D.M,cancer.
• Old age-cardiovascular disease

TIME DISTRIBUTION
• Short term fluctuations: Cholera, Measels
• Periodic fluctuations: Seasonal like respiratory
diseases in winter
• Long-term fluctuations: Cancer

5.Measurement of Disease
The amount of the disease ‘disease load’ in the
population.This information should be available in
terms of mortality, mobidity, disability and so on.
Measurement of Mortality is straightforward.
Morbidity has 2 aspects,
 Incidence
 prevelence

6. COMPARING WITH KNOWN INDICES
By making comparison between different populations,
by sub groups of the population. so we can identified
the increased risk for diseases.

7.FORMULATION OF ETIOLOGICAL
HYPOTHESIS
• By studying the distribution of disease to
formulate hypothesis relating the disease
aetiology.
E.g. cegirrate smoking causes lung cancer-
incomplete hypothesis.
• The smoking of 30-40 cigarettes per day
smoking causes lung cancer in 10%of smokers
after 20 years of exposure.

The case report is the presentation of the experience of
a single patient. A case report is a detailed report of the
symptoms , signs , diagnosis , treatment, and follow-up of an
individual patient.
Case reports may contain a demographic profile of the
patient, but usually describe an unusual or novel occurrence.
Some case reports also contain a literature review of other
reported cases.
Case reports are often referred to as Hypothesis-
generating because these bring forth evidence that supports
a Hypotheses or conclusion.
1) Case series/ Case report:
A.CaseReport:

B. CASESERIES:
When the common experiences of more than one
patient are presented, this is referred to as case series.
Greater the number of experiences stronger the
evidences.
EXAMPLE: if five patients developed aplastic
anemia due to the same medication, this would
raise questions. A good example is the case series
of 24 patients showing vuvular heart abnormalities
from concurrent fenfluramine which lead to its
withdrawal from the market

2) Cross-sectional studies
 Also known as prevelance study. It is the simplest form
of the observation study. Prevalence is the frequency of
cases at a given time.
They provide a snap shot of the frequency and
characteristics of a disease in a population at a particular
point in time
.
It is a single examination of a cross section of population at
one time and the results can be projected on the whole
population
Doesnot tell us about the history of the disease but only the
Ddistribution tell.

However since exposure & disease status are
measured at the same point in time, it may not be
possible to distinguish whether the exposure
proceeded or followed the disease and thus Cause
and effect are not certain. For example the study of
hypertension
Advantages;
Several outcomes
Short duration
Disadvantages:
Not feasible for rare diseases
Provide less information about the history of the
disease or the rate of occurance

It involves a repeated observation
of the same variables over longer period of time,
often many decades by means of follow-up
examination. Also known as INCIDENCE study.
Incidence is the development of
new cases in a population at risk.
 It is often used in psychology to study
developmental trends across the life span and
in sociology to study life events throughout
life time and generation.
Longitudinal studies

Longitudinal studies:
ADVANTAGES: 1. study the natural history of disease
2. Risk factors
3. Incidence rate
DISADVANTAGES:
1. difficult to organize
2. Time consuming

ANALYTICAL STUDIES
• The subject of interest is individual within the
population.
• 2 distinct types;
1.Case control studies
2.Cohort studies

Analytical epidemiology
Testing a specific hypothesis about a relationship of
a disease to a specific cause.
Analytical studies comprise 2 distinct types
1. Case control study
2. Cohort study

Case control study(retrospective study)
3 distinct features
 Both exposure and outcome have occurred
before the start of the study
 It uses a control/comparison group
 The study proceeds backwards from effect to
cause.
 Involves 2 populations
Cases Controls

Advantages
can obtain findings quickly
can often be undertaken with minimal funding
 efficient for rare diseases
Allows the study of several different aetiological
factors eg. Smoking, physical activity etc.
No attrition problems, because case control studies
donot require follow up of individuals into the future
 generally requires few study subjects
Disadvantages
cannot generate incidence data, can only estimate
relative risk
 subject to bias

COHORT STUDY
FEATURES ARE:
Cohorts are identified prior to the appearance
of disease under investigation.
Study groups are observed for a period of
time to determine the frequency of disease
among them.
Study proceeds forward from cause to effect

FRAMEWORK OF COHORT
STUDY
Study cohort: exposed to a particular factor
Control cohort: not exposed
Example: smokers and non smokers associated with lung
cancer
GENERAL CONSIDERATIONS:
1. The cohorts must be free from the disease
2. Both the groups should be equally susceptible to the
disease under study eg. Males over 35 years would be
appropriate for studies on lung cancer
3. Both the groups should be comparable in respect of all the
possible variables which may influence the frequency of
the disease

There are four basic steps in conducting a case
control study:
1. Selection of cases and controls
2. Matching
3. Measurement of exposure, and
4. Analysis and interpretation.

Advantages:
 Establish sequence of events
 Short duration
 Relatively cheap
 Can study several outcomes
 Dose response ratios can be estimated
Disadvantages:
 Often requires large sample sizes
 Not feasible for rare diseases
 Requires long period of follow up
 The study itself may alter people’s behaviour
 It is not unusual to loose a substantial proportion of
the original cohort,they may migrate or loose interest

Experimental Epidemiology
Experimental, where the epidemiologists have
control over the circumstances from the start it is
the study of the relationships of various factors
determining the frequency and distribution of
diseases in a community.
It provides a specific proof.
It can provide the strongest evidence for cause
and effect.

TYPES
• Randomised controlled trials.
• Non randomised or non experimental trails.

Randomized Controlled Trial (RCT)
(Synonym: Randomized Clinical Trial)
”An epidemiological experiment in which subjects
in a population are randomly allocated into
groups, usually called study and control groups
to receive and not receive an experimental
preventive or therapeutic procedure, or intervention”
John M.Last, 2001

Basic steps of RCT
1. The protocol
2. Selecting reference and
experimental populations
3. Randomization
4. Intervention
5. Follow up
6. Assessment

Eg; u.v rays of the sun effects on the skin
Select a suitable population
Select a suitable sample.
Make who are eligible/not eligible.
Randamize
Expermental control.
Manipulatio( or) intervention.
Fallow up.
Out come.

1. The protocol
- Rationale
- Aims and objectives, Research questions
- Design of the study: selection of study and
control groups
- Ethics: patient consent, adverse events
- Documentation
-Procedure
2. Selecting Reference and Experimental Populations
a. Reference or target population - population to which the
findings of the trial, if found successful, are expected to be
applicable (eg. drugs, vaccines, etc.)
b. Experimental or study population – actual population that
participates in the experimental study

Participants must fulfil the following criteria:
- Must give informed consent
- Should be representative of the
population
- Should be qualified or eligible for the trial

3. Randomization
- Heart of the control trial
- Procedure: Participants are allocated into study and
control groups
- Eliminates bias and allows comparability
- Both groups should be alike with regards to certain
variables that might affect the outcome of the
experiment
- Best done by using table of random numbers

4. Manipulation of Interventions:
Pharmaceutical (Therapeutic or Preventive)
Device
Procedure
Behaviour modification
5. Follow Up
- Implies examination of the experimental
and control group subjects
- at defined intervals of time,
- in a standard manner, with equal intensity,
under the same given circumstances

Randomised controlled trials design

USES
1 Tostudy the history of the disease
Trends of a disease for the prediction of trend
Results of studies are useful in planning for health services for public health
Community diagnosis
What are the diseases, conditions, injuries disorders, disabilities, defects causing
illness health problems, or death in a community or region.
risks of individuals as they affect populations What are the risk factors, problems,
behaviours that affect groups are studied by doing risk factor and disease
assessments
Assessment, evaluation and research
assessments: health screening , medical exams How well do public health and
health services meet the problems and needs of the population Effectiveness;
efficiency; quality; access; availability of services to treat, control or prevent
disease

5.Completing the clinical picture Identification and
diagnostic process to establish that a condition exists
or that a person has a specific disease Cause effect
relationships are determined,
e.g. strep throat can cause rheumatic fever
6. Identification of syndromes
Help to establish and set criteria to define syndromes,
some examples are: fetal alcohol, sudden death in
infants, etc.
7.Determine the causes and sources of diseases
Findings allow for control prevention, and elimination of
the causes of disease, conditions, injury, disability, or
death

EPIDEMIOLOGY

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