Epidemiology

PRESENTED BY:
Kinza Fatima
Anam Arshad
Ifra Zulfiqar
Amna Mohammad Afzal
Sidrah Yousaf Chaudary
Samra Zanjabeel

Epidemiology
(Study of epidemics)

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

DEFINITIONS:
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

 Other Definitions:
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.

History
The Greek physician Hippocrates is sometimes
said to be the father of epidemiology. He is the first
person known to have examined the relationships
between the occurrence of disease and
environmental influences. He coined the terms
endemic (for diseases usually found in some
places but not in others) and epidemic (for disease
that are seen at some times but not others).
One of the earliest theories on the origin of disease
was that it was primarily the fault of human luxury.
This was expressed by philosophers such as
Plato & Rousseau, and social critics like
Jonathan Swift

History
In the medieval Islamic world, physicians
discovered the contagious nature of infectious
disease. In particular, the Persian physician
Avicenna, considered a "father of modern
medicine," in The Canon of Medicine (1020s),
discovered the contagious nature of tuberculosis
and sexually transmitted disease, and the
distribution of disease through water and soil.
He also used the method of risk factor analysis,
and proposed the idea of a syndrome in the
diagnosis of specific diseases.

History
When the Black Death (bubonic plague) reached Al
Andalus in the 14th century, Ibn Khatima
hypothesized that infectious diseases are caused by
small "minute bodies" which enter the human body and
cause disease.
Another 14th century Andalusian-Arabian physician,
Ibn al-Khatib (1313–1374), wrote a treatise called On
the Plague, in which he stated how infectious disease
can be transmitted through bodily contact and "through
garments, vessels and earrings."

History
John Graunt, a professional haberdasher
and serious amateur scientist, published
Natural and Political Observations ... upon
the Bills of Mortality in 1662. In it, he used
analysis of the mortality rolls in London
before the Great Plague to present one of
the first life tables and report time trends for
many diseases, new and old.
He provided statistical evidence for many
theories on disease, and also refuted many
widespread ideas on them.

History
He used chlorine in an attempt to clean the
water and had the handle removed, thus
ending the outbreak. (It has been
questioned as to whether the epidemic was
already in decline when Snow took action.)
This has been perceived as a major event
in the history of public health and can be
regarded as the founding event of the
science of epidemiology.

History
In the middle of the 16th century, a famous Italian
doctor from Verona named Girolamo Fracastoro
was the first to propose a theory that these very
small, unseeable, particles that cause disease were
alive.
They were considered to be able to spread by air,
multiply by themselves and to be destroyable by fire.
In this way he refuted Galen's theory of miasms
(poison gas in sick people).
In 1543 he wrote a book “De contagione et
contagiosis morbis”, in which he was the first to
promote personal and environmental hygiene to
prevent disease.

History
The miasmatic theory of disease held that diseases
such as cholera or the Black Death were caused by a
miasma (Greek language: "pollution"), a noxious form
of "bad air". In general, this concept has been
supplanted by the more scientifically founded germ
theory of disease.

History
 Other pioneers include Danish physician P. A.
Schleisner, who in 1849 related his work on the
prevention of the epidemic of tetanus neonatorum on
the Vestmanna Islands in Iceland.
Another important pioneer was Hungarian physician
Ignaz Semmelweis, who in 1847 brought down infant
mortality at a Vienna hospital by instituting a
disinfection procedure.
His findings were published in 1850, but his work
was ill received by his colleagues, who
discontinued the procedure. Disinfection did not
become widely practiced until British surgeon
Joseph Lister 'discovered' antiseptics in 1865 in
light of the work of Louis Pasteur

History
In the early 20th century, mathematical methods were
introduced into epidemiology by Ronald Ross,
Anderson Gray McKendrick and others.

The Unique Contribution of
Epidemiology
Epidemiologic studies are conducted in
human populations
Lab and animal researches are incapable of
predicting the applicability of findings from a
particular species of animals to humans
Epidemiologic research provided
information for public health decisions long
before the basic mechanism of a particular
disease

EPIDEMIOLOGICAL DISEASES
Alphabetical Order
Anaplasmosis
Babesiosis
Chickenpox (Shingles or Varicella)
Corona virus
Diphtheria
Ehrlichiosis
Giardiasis

Haemophilius Influenzae
Hemolytic Uremic Syndrome (HUS)
Hepatitis A
Hepatitis B
Hepatitis C

Influenza
Measles
Mumps
Norovirus
Pertussis (Whooping Cough)
Q Fever
Rabies
Rubella (German Measles

Salmonellosis
Shingles (Varicella or Chickenpox)
Strep pneumoniae
Tetanus (Lockjaw)
Tuberculosis
Varicella (Chickenpox and Shingles)
Vector borne Disease
Vibrio parahaemolyticus

Common epidemic diseases in
Pakistan and mode of
transmission:
 food or waterborne diseases: bacterial
diarrhea, hepatitis A and E, and typhoid
fever
vectorborne diseases: dengue fever and
malaria
animal contact disease: rabies

TERMINOLOGIES
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

CONTINUE…..
 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

CONTINUE…
 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

CONTINUE…
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

CONTINUE…
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

CONTINUE…
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.

CONTINUE…
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

CONTINUE…
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

CONTINUE…
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

CONTINUE…
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.

CONTINUE…
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

CONTINUE…
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.

CONTINUE…
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

CONTINUE…
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

CONTINUE…
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

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

Components Of Epidemiology
Disease
frequency
Distribution of
disease
Determinants of
disease

Continue…
 The components of epidemiology
include the following:
Disease determinants - which are the
cause or factors, the Distribution, and
morbidity or mortality. Distribution
refers to how the disease occurs in the
population, Morbidity refers to
sickness, and Mortality refers to
death

1. DISEASE FREQUENCY
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

INCIDENCE
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

2.DISTRIBUTION OF
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

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

3. DETERMINANTS OF
DISEASE
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.

Epidemiological triad
HOST
AGENT
VECTOR
ENVIRON
MENT

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.

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
• 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

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)
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
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

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 reservior
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

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
• 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
Verticle 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

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.

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

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)
can also come from the introduction of already produced
antibodies into a susceptible case

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

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

Study Design:
Study design is a specific plan or protocol
for conducting a study which allows the investigator
to translate the conceptual hypothesis into an
operational one.
Hypothesis:
It is an educated guess about an association
that is testable in a scientific investigation

OBSERVATIONAL STUDIES
Descriptive epidemiology
Case reports /case series
Cross sectional studies
Longitudinal studies
Analytical epidemiology
Case control
Cohort studies
EXPERIMENTAL STUDIES
Clinical 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:
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?)
Basic Triad of Descriptive Epidemiology
THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE
WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY ARE:
PERSON
PLACE
TIME

 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.
 It is the first phase in the investigation

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

Defining the
Population
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
do not.

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

TIME DISTRIBUTION
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

PLACE DISTRIBUTION
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

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

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

1) Case series/ Case report:
A.Case Report:
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.
EXAMPLE The presentation of the medications for the patient
that were administered until the development of aplastic anemia
suggest that 1 of these may have caused it. However it is not
concluded that another patient taking the same could be at risk
because of many other factors eg insecticides, viral infection which
may not b a part of the medical report

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
first to provide clues in identifying a new disease
or adverse health effect from an exposure.

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.
Much like a cine film. 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: 1. difficult to organize
2. Time consuming
For example the study of bp in a community will reveal
the normal values rather than the abnormal ones
related to the disease

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
to cause

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
subject to bias

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 single

Experimental
Epidemiology:
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:
Trial
Controlled Not controlled
Randomised Not randomised
Blinded Not blinded

Randomized control
trials(RCT):
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

THE PROTOCOL:
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

USES:
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

Epidemiology

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