Epidemiology newwwwwwwwwwwwwwwwwwww.pptx

 The word “epidemiology” derives from
GREEK word
 Epi ( Upon)
 Demons(people)
 Ology(science)
 So it deals with what fall among people.

A Modern Definition….
According to WHO
 Study of the occurrence and distribution of
health-related diseases or events in specified
populations, including the study of the
determinants influencing such states, and the
application of this knowledge to control the
health problem

 Epidemiology is the study of the distribution
and determinants of health-related states or
events (including disease), and the application
of this study to the control of diseases and
other health problems.

 Study
 Distribution (descriptive epidemiology) – The
who, what, where, when, and how many?
 Determinants (analytic epidemiology) – The
how and why?
 Health-related states or events
 Specified populations
 Applications

 Science of community medicine that deals with
study of distribution, determents and
frequency of disease in population is known as
Epidemiology
 OLDER DEFINITION:
 The study of the occurrence of illness.

 food or waterborne diseases: bacterial
diarrhea, hepatitis A and E, and typhoid
fever
 vectorborne diseases: dengue fever and
malaria
 animal contact disease: rabies

Endemic
•When an infectious
disease more or
less prevailing on a
locality or
community called
as endemic
•E.g.. Chicken pox
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
 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 extent of pathogenicity or strength of different
organisms
 the ability of the pathogen to grow, thrive, and to
develop all factor into virulence
 the capacity and strength of the disease to produce
severe and fatal cases of illness
INVASSIVNESS:
the ability to get into a susceptible host and cause a
disease within the host
 The capacity of a microorganism enter into and grow
in or upon tissues of a host

ETIOLOGY:
The factors contributing to the source of or causation of
a disease
TOXINS:
A poisonous substance that is a specific product of the
metabolic activities of a living organism and is usually
very unstable
 notably toxic when introduced into the tissues, and
typically capable of inducing antibody formation

HYPERENDEMIC:
Diseases that affect a high proportion of population at
risk.
HOLOENDEMIC:
Disease that is highly prevalent in a population & is
commonly acquired early in life in most all of the
children of the population
MESOENDEMIC:
Diseases that affect a moderate proportion of population
at risk.

HYPOENDEMIC:
Diseases that affect a small proportion of population at
risk.
SPORADIC:
A Disease that is normally absent from a population but
which can occur in that population, & although relay &
without predictable regularity.
INCIDENCE:
the extent that people, within a population who do not
have a disease, develop the disease during a specific
time period.

PREVALENCE:
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.
ANTIBIOTICS:
Substance produced by or a semisynthetic substance
derived from a microorganism &able in dilute solution
to inhibit or kill another microorganism

AGENT: is the cause of the disease
 Can be bacteria, virus, parasite, fungus, mold
 Chemicals (solvents), Radiation, heat, natural toxins
(snake or spider venom)
HOST:
is an organism, usually human or animal, that
harbors the disease
PATHOGEN:
disease-causing microorganism or related
substance
ENVIRONMENT:
is the favorable 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.
CONTAMINATION:
It is the presence of infectious agent in or upon
the surface of articles, wound or inanimate
object such as cloth, toys, bed, floor etc.
CONTAGIOUS DISEASE:
A disease which is transmitted by contact.

COMMUNICABLE DISEASE:
the disease which is transmitted from one
person to another directly or indirectly through
infectious agent like Food, Air, Water, Dust is
called communicable disease.
NON COMMUNICABLE DISEASE:
The diseases which are not transfer to another but
they occur within the patient himself e.g.
cancer, diabetes, hypertension

FOMITES:
Inanimate articles other than food& water ,
contaminated by infectious discharge from the
patient& are capable of transmitting the
infectious agent to the healthy person e.g cloth,
towel, handerkerchief.
CARRIER:
one that spreads or harbors an infectious
organism

LATENT PERIOD:
The time from first contact with nonliving agent until
symptoms appear (cold, heat, irradiation, poisons,
toxins, etc.).
EPIZOOTIC:
An condition of outbreak of disease in animals
POLLUTION:
The existence of certain abnormal amounts of
toxic chemicals or dust within an
environmental category such as air, water,
food, or soil

CLINICAL INFECTION:
The state in which the host has symptoms, feels
ill, or dies. Clinical infection and disease are
terms often used as synonyms.

Disease
frequency
Distribution of
disease
Determinants of
disease

 Rate & Ratio, analysis of the incidence & the
prevalence of a disease. There are two main
measures of disease frequency
A) PREVALENCE
B) INCIDENCE
•Quantification of the existence or occurrence of
disease

The probability that healthy people will develop
a disease during a specified period of time (that
is, the number of new cases of a disease in a
population over a period of time). Incidence
measures the rapidity with which a disease
occurs or the frequency of addition of new
cases of a disease. These new cases of disease
occur either through onset of the disease in
current

Members of the population or by
immigration into the population of
persons already ill. The formula for
determining incidence rates is:
Incidence rate = No. of new cases during a given period ×
10n Population at risk
during the same period
 PREVALENCE:
 The number of people in a population
who have a given disease at a given
period of time.
 The formula for determining
prevalence rates is:

 Prevalence =
All new & preexisting cases during a given time period ×10n
Population at risk during the same time period
Note: It is important to remember that the
rates for both incidence and prevalence
include a factor of 10 such as per 100 or
per 1,000. (Rate is usually expressed per
1,000.) The value of n depends on the
relative frequency of a given disease

 Who is getting the disease within a population
 Where and when the disease is occurring
 To describe patterns of disease as well as to
formulate hypotheses concerning causal or
preventive factors

PERSON TIME PLACE
Age Point
epidemic
Geographic
Race cyclical Longitude&
latitude
Sex Secular Geologic
Occupation Climatic
Education Geo
Political
Hobbies Urban/Rur
al
Industry
Pollution

By this we mean the cause of a disease. It includes :
PRIMARY DETERMINENTS
Primary cause of disease.
SECONDARY DETERMINENTS
Factors responsible for the spread of the disease.

HOST
AGENT
VECTOR
ENVIRON
MENT

 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.

 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.

 It includes
• bilogical 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

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

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)
MECHANICAL AGENTS
 Injury
 Accidents
 Machinery

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 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)

 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

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.

 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

 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)

 Also called zoonoses
 An animal become reservior 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

 Includes soil , water etc
 Soil contains bacteria which cause tetnaus
 Water contains micro organism(protoza)
causing different diseases like malaria
dengue….

• 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

 DIRECT
Direct contact
Verticle transmission
Droplet infection
Animal bite transmission
Contact with soil

 INDIRECT
Airborne
Vehicle borne
Vector borne
Formite borne
Hand borne

• 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

 pathogens or agents are transferred or carried
by some intermediate item or organism, means
or process to a susceptible host

Indirect transmission
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
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.

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

3. 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:
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:

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.

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:
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 off 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
 can occur through a vaccine or in response to having a
similar disease
 Similar to acquired
 Passive Immunity (natural passive) acquired
through transplacental transfer of a mother’s
immunity to diseases to the unborn child (also via
breastfeeding)

When there is
little to no
immunity
within a
population,
the disease
spreads
quickly

Herd
Immunity
the resistance
a population
or group
(herd) has to
the invasion
and spread of
an infectious
disease

 Anthrax
 Chicken pox
 Cholera
 Diphtheria
 German measles
(rubella)
 Hepatitis A & B
 Influenza
 Malaria (in process)
 Measles
 Meningitis
 Mumps
 Plague
Diseases for which vaccines are
used
• Pneumonia
• Polio
• Rabies
• Small pox
• Spotted fever
• Tetanus
• Tuberculosis
• Typhoid Fever
• Typhus
• Whooping Cough
• Yellow Fever

*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:
In a descriptive study, the epidemiologist
collects information to characterize and
summarize the health event or problem.
It is limited to the description of the
occurrence of a disease in a population

in the descriptive process, we are
concerned with
"person" (Who was affected?)
"place" (Where were they affected?)
and time (When were they
affected?)
THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE
WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY ARE:
 PERSON
 PLACE
 TIME
Basic Triad of Descriptive Epidemiology

 Descriptive epidemiology study the
patterns or trends in a situation but not a
cause an effect linkages among different
elements.
 Examining the distribution of a disease
in a population, and observing the basic
features of its distribution in terms of time,
place and person.
 It helps in the generation of hypothesis.
 Measurement of disease in terms of
mortality, morbidity, disability.

Procedure:
The procedure involve in such studies includes
Defining the population to be studied
Defining the disease under study
Describe the distribution of disease in relation to Time
Place & Person
Measurement of disease in terms of mortality
,morbidity and disability
Finally formulation of etiological hypothesis

1) 1.Descriptive studies are investigations of
populations not individuals
2) 2.The defined population can be:
o The whole population
o A representative sample
DEFINING THE DISEASE UNDER STUDY:
The epidemiologist whose main concern is to
obtain an accurate estimate of the disease in
a population needs a definition that is both
precise and valid to enable him to identify
those who have the disease from those who

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,weig
ht
duration Towns, cities Occupation,
social status,
education
BP, blood
cholestrol,
personal
habbits

The pattern of disease may be described by
the time of its occurance, i.e week, month,
year, day of week etc.
Epidemiologists have identifies three kinds of
time trends or fluctuations
1. Short term fluctuations
2. periodic fluctuations
3. Long term fluctuations

By studying the distribution of the disease in
different populations we gain perspective in
disease patterns not only between countries
but also within countries. Geographic
patterns provide the causes of the disease.
a. International variations
b. National variations
c. Rural-urban differences
d. Local distributions

The disease is further characterized by
defining the person who develops the disease
by various factors:
 Age
 Sex
 Ethnicity
 Martial status
 Occupation
 Stress
 migration

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

B. CASE SERIES:
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

Most case reports are on one of six topics:
i. An unexpected association between
diseases or symptoms .
ii. An unexpected event in the course of
observing or treating a patient.
iii. Findings that shed new light on the
possible pathogenesis of a disease or an
adverse effect.
iv. Unique or rare features of a disease.
v. Unique therapeutic approaches.
vi. A positional or quantitative variation of the
anatomical structures.
Advantage:
Case series/Case report may be the

2) Cross-sectional studies:
 Also known as prevelance study. It is the
simplest form of the observation study.
Prevelence 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
distribution
This type of a data can be used to assess the prevalence of
acute or chronic condition in a population. But mostly for
chronic
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

Longitudinal studies:
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.
ADVANTAGES:
1. study the natural history of disease
2. Risk factors
3. Incidence rate
DISADVANTAGES:

FORMULATION OF HYPOTHESIS
By studying the distribution of the disease and
utilizing the techniques of descriptive
epidemiology, it is possible to formulate
hypotheses.
Example: ‘ cigerrete smoking causes lung
cancer”. This is incomplete hypothesis
Complete hypothesis:
“ the smoking of 30-40 cigarettes per day
causes lung cancer in 10 percent of the
smokers after 20 years of exposure”

Advantages of descriptive epidemiology:
It provides clues of etiology of disease.
Provide data regarding magnitude and type
of disease problems in community in terms of
morbidity ,mortality ,rates & ratios.
Background data for planning ,organizing
,preventive and curative services
Contribute to reasearch by describing
variations in the disease occurance by time,
place and person

ANALYTICAL EPIDEMIOLOGY
In analytical studies the subject of interest is the
individual within the population. The object is
not to formulate but to test hypothesis.
Once we know the answers to the questions in
descriptive epidemiology, we can enter the
realm of analytical epidemiology and ask how
and why these people were affected.
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
It refers to as Retrospective studies and they
serve as first approach to test any casual
hypothesis.
Emerged as the permanent method of
epidemiology
Case control studies are often used to identify
factors that may contribute to a medical condition
by comparing subjects who have that
disease(cases) with patients who don’t have that
disease(control group) but are otherwise similar.
The control group should ideally come from the
same population.
Case control studies has different features:
 Both the exposure ( cause) an outcome (effect)
have occurred before the study is taken up.
 The study proceeds backwards from the effect

Case  individuals with particular disease
Control  individuals without particular disease
BASIC STEPS
1. Selection of cases and controls
2. Matching
3. Measurement of exposure and
4. Analysis and interpretation
BIAS:
Systemic error in the determination of the association
between exposure and disease
Bias due to memory recall
Selection bias
Interviewers bias

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

COHORT STUDIES
Cohort:
Cohort can be defined as a group of people
which shares a common characteristics or experience
with in a defined time. Eg.
Birth cohort: group of people born on the same day
Exposure cohort : persons exposed to a common
drug or vaccine
DISTINGUISHING FEATURES:
1. Cohorts are identified prior to the appearance of
the disease under investigation
2. The study groups, so defined, are observed over a
period of time to determine the frequency of
disease among them
3. The 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

ELEMENTS OF COHORT:
1. Selection of study subjects
2. Obtaining data on exposure
3. Selection of comparison groups
4. Follow up
5. analysis

TYPES OF COHORT STUDIES
PROSPECTIVE COHORT STUDIES:
More preferred type of study but expensive.
“a prospective or current cohort is one in which the
outcome(disease) has not yet occurred at the time the
investigation begins”
Begin in the present and continue into future
EXAMPLE:
Study cohort: uranium miners
Control cohort: non-miners
Disease: lung cancer
The principal finding was that the uranium miners
had an excess frequency of lung cancer campared to
non-miners. since the disease had not yet occurred
when the study was undertaken this is the
prospective cohort design

RETROSPECTIVE COHORT STUDIES
also known as historical cohort study
Exposure and outcome have already occurred at the start of the
study. Pre-existing data, such as medical notes, can be used to
assess any causal links, so lengthy follow-up is not required.
This type of cohort study is therefore less time consuming and
costly, but it is also more susceptible to the effects of bias. For
example, the exposure may have occurred some years
previously and adequate reliable data on exposure may be
unavailable or incomplete. In addition information on
confounding variables may be unavailable, inadequate or
difficult to collects
* More economical and produce results more quickly than
prospective cohort studies

COMBINATIONS OF PROSPECTIVE AND RETROSPECTIVE
COHORT STUDIES:
In this type of study these elements are combined
For example: patients who received large doses of
radiation therapy for ankylosing spondylitis. The
outcome was death due to aplastic anemia. They
found that the death from aplastic anemia was higher
in their cohort than the general population. Thus a
prospective component was added to identify deaths
in the subsequent years

 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

Strengths in Cohort vs. Case-control?
 Cohort study :
• Rare exposure
• Examine multiple effects of a single exposure
• Minimizes bias in the in exposure
determination
• Direct measurements of incidence of the
disease
 Case-control study :
• Quick, inexpensive
• Well-suited to the evaluation of diseases with
long latency period
• Rare diseases
• Examine multiple etiologic factors for a

Experimental, where the epidemiologists have
control over the cicumstances 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 Of Trials:
Blinded Not blinded
Randomised Not randomised
Controlled Not controlled
Trial

It is a specific type of scientific experiment.
It is used to study a particular intervention.
Subjects in the study population are
randomly allocated to intervention and
control groups, and the results are assessed
by comparing the outcome.
Basic steps:
Drawing a protocol
Selecting a reference and experimental populations
Randomization
Manipulation or intervention
Follow up
Assessment of outcome

 Protocol specifies the aims and objectives of the
studies,criteria for the selection of the study and control
groups,size of the sample,procedures for a location etc.It
aims at preventing bias and to reduce the sources of
errors in the study.
SELECTING REFERANCE AND
EXPERIMENTAL POPULATION:
a. Refererence population or target population is
the population to which the findings of the
trial if found successful are applicable eg.
Drug, vaccine etc
b. Experimental or study population is derived
from the reference population. The actual
population that participates in the
experimental study

CRITERIA:
a. Informed consent
b. Representative of the population
c. Eligible of the trial
RANDOMIZATION:
Statistical procedure by which the participants are
allocated into groups usually called “study” and
“control” to receive or not to receive the
experimental, preventive or therapeutic procedure
Attempt to eliminate bias and to ensure that the
investigator has no control over the allocation
The essential difference between a randomized
control trial and an analytical study is that in the
latter, there is no randomization because
differentiation into diseased and non-diseased
groups has already taken place

MANIPULATION:
After formation of the groups the next step is to
intervene or manipulate the study group by the
deliberate application or withdrawal or the reduction
of the suspected casual factor e.g. drug vaccine etc.
FOLLOW-UP:
This implies the examination of the experimental and
the control groups at defined intervals of time, in a
standard manner, with equal intensity
Some loses to follow up are inevitable due to death,
migration and loss of interest. This is known as
attrition

ASSESMENT:
positive and negative results are deuced
Sequential analysis may be done
BIAS: may arise from the errors of assessment of the
outcome due to human element. There are 3 types:
a. Subject variation: bias on the part of the
participants who may subjectively feel better or
report improvement if they knew they were
receiving new form of treatment
b. Observer bias: observer measuring the outcome
may become influenced
c. Bias in evaluation: investigator may give a
favorable report of the outcome

BLINDING:
1. Single blind trial: the trial is so planned that the
participant is not aware whether he belongs to the
study group or control group
2. Double blind trial: the trial is so palnned that
neither the doctor nor the particioant is aware of
the group allocation and the treatment received
3. Triple blind trial: the participant, investigatior
and the person analysing the data are all blind

SOME STUDY DESIGNS:
1. concurrent parallel study designs: one group is
exposed to the treatment and the other grop is not
exposed
2. Cross over type of study design: the study group
receives the treatment under consideration and
the control receives the alternate, placebo
Cannot be used if
a. It cures the disease
b. Only effective on a certain stage

TYPES OF RANDOMIZED CONTROL
TRIALS:
1. Clinical trials
2. Preventive trials
3. Risk factor trials
4. Cessation trials
5. Trial of aetiological agents

NON-RANDOMIZED CONTROL TRIALS
In non randomized controlled trials, the control
group is predetermined (without random
assignment) to be comparable to the program
group.
Because the study groups are opportunistically
rather than randomly composed, study group
characteristics (age, sex) may not be balanced
before (at baseline) the study begins.
1. Uncontrolled Trials: trials with no comparison
group

2. NATURAL EXPERIMENTS:
Where the experimental studies are not possible in
human populations the epidemiologist seeks to
identify “natural circumstances” that mimic an
experiment. For example:
1. smokers and non-smokers have naturally
separated themselves into two groups
2. cholera

1 To study the history of the disease
Trends of a disease for the prediction of trend
• Results of studies are useful in planning for health
services and public health
2. Community diagnosis
 What are the diseases, conditions, injuries, disorders,
disabilities, defects causing illness, health problems, or
death in a community or region
3. Look at risks of individuals as they affect populations
 What are the risk factors, problems, behaviors that
affect groups
 Groups are studied by doing risk factor assessments:
health screening , medical exams and disease
assesments
4. Assessment, evaluation and research
 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

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