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• Epidemiology is derived from Greek word
EPI – upon
DEMOS – people
LOGOS – study
• Epidemiology means the study of disease in
• 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.
CONCEPT OF EPIDEMIOLOGY
PLATO AND ROUSEAU: “FAULT OF HUMAN LUXUARY”
HIPPOCRATES: “ENVIRONMENTAL INFLUENCES”
IBN KHATIMA: “SMALL MINUTE BODIES”
Definition of Epidemiology
The study of frequency, distribution and
determinants of diseases is known as
(International epidemiological association)
“Epidemiology is defined as the study of the
distribution and determinants of health related
states or events in specified populations and the
application of this study to control the health
LAST-(September 22, 1926 - September 11, 2019)
Key Words of Epidemiology
• Epidemic : ( Epi = upon : demos = people ) An
outbreak of disease in a community in excess of
“normal expectation ”
• Endemic: (En = in; demos = people). The
constant presence of disease within a geographic
area or the usual prevalence of a given disease in a
particular area. malaria, tuberculosis, etc.
• Pandemic: (Pan = all: demos = people) An
epidemic which spreads from country to country or
over the whole world, as for example, the recent
Aims of Epidemiology
• To describe the distribution and size of disease
problems in human population.
• To identify etiological factors in the
pathogenesis of disease.
• To provide data essential to the planning,
implementation, and evaluation of services for
the prevention, control and treatment disease and
to setting up the priorities among these services.
1. ACCORDING TO DISEASEGROUP
INFECTIOUS DISEASE EPIDEIOLOGY:
CARDIO VASCULAR EPIDEMIOLOGY:
2. ACCORDING TO GROUP OFFACTORS
• NUTRITIVE EPIDEMIOLOGY:
• REPRODUCTIVE EPIDEMIOLOGY:
• ENVIRONMENTAL EPIDEMIOLOGY:
• CLINICAL EPIDEMIOLOGY
• MOLECULAR EPIDEMIOLOGY
• GENETIC EPIDEMIOLOGY etc.
Search of cause / causes of diseases
Helps to describe the health status of population
Helps to discover and bridge gaps in natural history of
Helps in controlling diseases
Helps in better management of health services
To study the effects of disease state in a population over a time
and predict future health.
To diagnose the health of the community.
To evaluate health services.
To estimate the individual risk from group experiences.
To identify the syndrome.
To search for causes of disease.
USE OF EPIDEMIOLOGY
LEVELS OF PREVENTION
• Health promotion
• Early diagnosis (screening tests, case finding programmes )
• Disability limitation
– Functional rehabilitation
– Vocational rehabilitation
– Social rehabilitation
– Psychological rehabilitation
1. Asking questions
2. Making comparisons
1. Asking questions
• Epidemiology has been defined as a means of
• learning or asking questions.. and getting answers that lead to
• RELATED TO HEALTH EVENTS:
•What is the event? (the problem)
•Where did it happen?
•When did it happen?
•Who are affected?
•Why did it happen?
RELATED TO HEALTHACTIONS
•What can be done to reduce this problem and its
•How can it be prevented In the future ?
•What action should be taken by the community ? By
the health services? By other sectors ?
•What resources are required ? How are the activities to
be organized ?
2. Making comparisons
•This approach is to make comparisons and draw
•Comparison may be made between different
population at a given time eg. Rural with urban
•Between sub group of population eg. Male with
•Between various periods of observation eg.
A rate measures the occurrence of some particular
event (development of disease or the occurrence of
death) in a population during a given time period.
Number of deaths in one year
Death rate = X 1000
Mid - year population
(1) Crude rates: These are the actual observed rates
such as the birth and death rates. Crude rates are also
known as unstandardized rates.
(2) Specific rates: These are the actual observed
rates due to specific causes (e.g., tuberculosis); or
occurring in specific groups (e.g., age-sex groups) or
during specific time periods (e.g.. annual, monthly or
(3) Standardized rates: These are obtained by
direct or indirect method of standardization or
adjustment, e.g., age and sex standardized rates.
Another measure of disease frequency is a
ratio. It expresses a relation in size between two
•The number of children with malnutrition at a
•sex-ratio, doctor population ratio, child woman
A proportion is a ratio which indicates the relation in
magnitude of a part of the whole. The numerator is always
is usuallyincluded in the denominator. A proportion
expressed as a percentage.
The number of children with
scabies at a certain time
Example x 100
The total number of children in
the village at the same time
The occurrence of death in a population.
Mortality data are relatively easy to obtain,
and, in many countries, reasonably accurate. Many
countries have routine systems for collecting
mortality data each year, information on deaths is
analyzed and the resulting tabulations are made
available by each government.
Limitations of mortality data
• Incomplete reporting of deaths
• Lack of accuracy:-inaccuracies in the recording of
age and cause of death
• Lack of uniformity:- There is no uniform and
standardized method of collection of data
• Changing:- Changing coding systems and changing
fashions in diagnosis may affect the validity.
• Diseases with low fatality:-the disease is associated
with low fatality (e.g., mental diseases, arthritis).
1. Crude death rate
The simplest measure of mortality is the crude
death rate It is defined as the number of deaths
(from all causes) per 1000 estimated mid-year
population in one year, in a given place.
Number of deaths during the year
= X 1000
2. Specific death rates
When analysis is planned to throw light on
aetiology, it is essential to use Specific death
rates. The specific death rates may be
(a)cause or disease specific e.g., tuberculosis
(b)related to specific groups e.g. age-specific,
sex-specific , age and sex specific, etc.
3. Case fatality rate (Ratio)
Case fatality rate represents the killing power of a
disease.it is simply the ratio of deaths to cases. The time
interval is not specified. Case fatality rate is typically used
in acute infectious diseases (e.g., food poisoning, cholera,
Total number of deaths due to
a particular disease
= X 100
Total number of cases due to
the same disease
4. Proportional mortality rate (Ratio)
•It is sometimes useful to know what proportion of total
deaths are due to a particular cause (e.g.. cancer) or
•what proportion of deaths are occurring in a particular
age group (e.g., above the age of 50 years).
•Proportional mortality rats expresses the number of
deaths due to a particular cause (at in a specific age
group) per 100 (or 1000) total deaths. Thus we have:
(a) Proportional mortality from a specific disease
Number of deaths from the specific
disease in a year
= X 100
Total deaths from all causes
in the year
2. Under 5 proportionate mortality rate
Number of deaths under 5 years of age
in the given year.
= X 100
Total number of deaths during
the same period
3. Proportional mortality rate for aged 50 years and above
Number of deaths at persons aged 50
years and above
= X 100
Total deaths at all age groups
in that year
It is the proportion of survivors in a group, (e.g. of
patients) studied and followed over a period tag, a 5 year period.
Survival rates have received special attention in cancer studies.
Total number of patients alive
after 5 years
Survival rate = X 100
Total number of patients diagnosed
The occurrence of an illness or illnesses in a
Morbidity has been defined as any departure, subjective or
objective, from a state of physiological well-being. The term
is used equivalent to such terms as sickness. illness,
WHO Expert Committee on Health Statistics noted in
its 6thReport that morbidity could be measured in terms of 3
units (a) persons who were ill
(b)the illnesses (periods or spells of illness) that these
persons experienced; and
(c) the duration (days, weeks, etc.) of these illnesses
Incidence rate is defined as the number of NEW
cases occurring in a defined population during a specified
period of time . It is given by the formula :
Number of new cases of specific disease
during a given time period
Incidence = X 1000
Population at risk during that period
For example:- if there had been 500 new cases of an illness
in a population of 30,000 in a year, the incidence rate would
= 500/30,000 x 1000
= 16.7 per 1000 per year
The term "disease prevalence" refers specifically
to all current cases (old and new) existing at a given
point in time, or over a period of time in a given
(a) Point prevalence (b) Period prevalence
(a) Point prevalence
•Point prevalence of a disease is defined as the number
of all current cases (old and new) of a disease at one
point of time, in relation to a defined population. several
days, or even a few weeks.
Point prevalence is given by the formula:
Number of all current cases (old and new) of a
specified disease existing at a given point in time
Estimated population at the same point in time
(b) Period prevalence
A less commonly used measure of prevalence is period
prevalence. It measures the frequency of all current cases (old
and new) existing during a defined period of time (e.g., annual
prevalence) expressed in relation to a defined population. cases
arising during the year.
Period prevalence is given by the formula:
Number of existing cases (old and new) of a specified
disease during a given period of time interval
Estimated mid interval population at risk
Epidemiologists employ three different• Epidemiologists employ three different
methods or approaches for epidemiological
studies which are:
1. Descriptive method
2. Analytical method.
3. Experimental method
Descriptive method of epidemiological study
is concerned with the study of frequency and
distribution of disease and health related events
in population in terms of person, place and time.
This method gives information about which
all are affected by a particular disease or health
related event or problems, where the cases occur
and when they occur. The data is collected about:
• Personal characteristics such as age, sex, race,
marital status, occupation, education, income,
Class, dietary pattern, habits.
• Place distribution of cases i.e. areas of high
concentration, low concentration and spotting in
• Time distribution/trends such as year, season,
month, week, day and hour of onset of the
Such information’s give clue to possible
associated factors such as age with specific
disease e.g. measles, diphtheria, pertusis in early
childhood, cancer in middle age, arterioscleroses
in old age, some habits like smoking with lung
cancer, dietary pattern with obesity. The data
collected are analyzed and presented in terms of
percentages, rates and ratios.
In this design of descriptive method of study, the data is
collected from a cross-section of population at a one
point in time.
The results of the study are applied on the population. The
cross-section of the population is sampled carefully so
that it is representative of the whole population.
Cross-section study is like a snapshot and provides
information about the prevalence of a disease.
It is also called as prevalence study. example study of
diabetes or hypertension by personal characteristics and
Longitudinal studies art useful for studying
the natural history of diseases, finding out
incidence rates of diseases and identifying risk
factors of diseases. Longitudinal studies are
more expensive and time consuming than cross-
You have learn that descriptive studies
generate etiological clues for various diseases
help in formulating a guess or hypothesis for
further vigorous study or testing e.g. "Cigarette
smoking (10 to 20 in a day) causes lung cancer in
10 to 15% of smokers after 20 years of
exposure”, wife battering is related to violence in
victim's childhood family of origin". These types
of hypothesis are further studied and tested by
analytical studies to determine the association of
cause with the effect.
- Case control study
In this method a group of people who have
been diagnosed as having a particular problem
e.g. lung cancer (cases) are compared with a
group of people who are similar in
characteristics to that of cases but they are free
from the problem i.e. free from lung cancer
under study (controls).
- Cohort Study
A cohort is a specific group of people, at a
certain time, sharing common characteristics or
experience e.g. people born on the same day or
the same year (birth cohort), couple married in
the same year (marriage cohort), a class of
nursing students (experience cohort), people
With same occupation (occupation cohort) etc.
a) Prospective cohort study: Cohort study is
prospective in nature because the group under study
is free from the disease but exposed to risk factor
and epidemiologist study the development of a
condition over time. example-lung cancer
b) Retrospective cohort study: This is also called as
historical cohort study. In this type of study designs
the event has already occurred
• Experimental studies are similar in approach to cohort
studies except that conditions are under the careful
control of investigator.
• Experimental studies are done to confirm the etiology of
diseases,' establish the efficacy of preventive or
therapeutic measures and evaluate health care services.
• These' studies are done under controlled conditions.
• The investigator administers an intervention/gives
treatment to the experimental group which is either
exposure to causative factor of disease or
preventive/therapeutic measure to improve or influence
health or prevent disease but not to the control group
which is similar to experimental group in all its aspects.
2nd: Source or Reservoir
• The starting point for the occurrence of a communicable
Source of infection :•
• the person, animal, object or substance from which an
infectious agent passes or is disseminated to the host
• “any person, animal, arthropod, plant, soil, or
substance, or a combination of these, in which
an infectious agent normally lives and multiplies,
on which it depends primarily for survival, and
where it reproduces itself in such a manner that
it can be transmitted to a susceptible host.
It is the natural habitat of the infectious agent.” 4
Types of Reservoirs
“A person in the populationor
Identifiedas having particular
disease, health disorder or
• Under investigation”
The clinical illness maybe mild or moderate,
typical or atypical, severe or fatal.
Epidemiologically, mild cases may be
more impor tant sources of
infection than severe cases because they
are ambulant and spread the infection
wherever they go, whereas severe cases
usually confined to bed.
Inapparent, Covert, Missed or Abortive Cases
• disease agent multiply in the host but does
not manifest by S/S.
• But contaminates the environment in the
same way as clinical cases.
• Subclinical cases play a dominant role in maintaining the
chain of infection in the community.
detected only by
occurs in most
Hepatitis A and B,
• infectious agent lies iinn aa nonon-n-iinfnfectectiioousus
ddormormantant within the host without
sysymmptptoommss with no shshededdidinngg (and often
without demonstrable presence in blood,
tissues or bodily secretions of the host)
HSV and VZV: nerve ganglia cells,
CMV: kidney and salivary glands cells,
◦ Person that comes to the
attention of public health
◦ First case of a communicable
disease introduced into the
population unit bring studied
◦ Attack rate
◦ Person who acquiresthe disease
from an exposure to the primary
◦ Secondary attack rate
due to inadequate treatment or immune respons
the disease agent is not completely eliminated
leading to a carrier state.
An Infected Person or Animal that harbours a specific
infectious agent in the absence of discernible (Visible)
clinical disease and serves as a potential source of
infection to others
• As a rule carriers are less infectious than
cases, but epidemiologically, they are more
dangerous than cases
◦ escape recognition
◦ continuing to live a normal life among population
◦ readily infect the susceptible individuals
◦ over a wider area and longer period of time under
IncuIncubatbatoorryy CCaarrrrieierrs:s: those who shed
the infectious agent during the incubation
period. This usually occurs during last few days
Measles- the period of communicability is 4 days before the rash.
Mumps- usually 4-6 days before onset of symptoms
Polio- 7-10 days before onset of symptoms
Hepatitis B- for a month before jaundice
Classification of Carrier
Carrier May Be
those who continue to shed the disease agent during
the period of convalescence
In the disease, clinical recovery does not
coincide with bacteriological recovery.
Serious threat to HH members
Highlights importance of bacteriological surveillance
of carriere state after recovery
◦ typhoid fever
◦ bacillary dysentery
Carrier may be
victims of subclinical infection who have
developed carrier state without suffering from
overt disease, but are nevertheless shedding the
◦ meningococcal meningitis,
Note:- Person whose infection remains subclinical may or may not act as
carrier (eg.- in polio inf may remain subclinical but person act as temp carrier
due to shedding of virus in stool..while TB most of us with +ve Mt, do not
disseminate bacillie- so not labelled as carrier.
Temporary carriers are those who shed the
infectious agent for short period of time.
Chronic carriers are those who excretes the
infectious agent for indefinite periods
Chronic carriers are far more important sources of
infection than cases.
The longer the carrier state, the greater the risk of
community-- reintroduce disease into areas which
are otherwise free of infection
The duration of the carrier state varies with the
In typhoid fever and hepatitis B, the chronic
carrier state may last for several years.
In chronic dysentery it may last for year or
In diphtheria, the carrier state is associated with
infected tonsils, in typhoid fever with gall bladder
Mary Mallon (1869 –1938), better
known as Typhoid Mary, was the
first person in the US identified as an
asymptomatic carrier of the
pathogen associated with
She was presumed to have
infected some 50 people, three of
whom died, over the course of her
career as a cook.
She was forcibly isolated twice by
public health authorities and died
after a total of nearly three
decades in isolation.
Respiratory carrier: e.g.influenza
Fecal (intestinal) carrier: e.g. typhoid,
Blood carrier: e.g. hepatitis B andHIV
Urinary : e.g.Typhoid
sexual Carrier: gonococcus and HIV
By Portal Of Exit of Infectious Agent
• infection that is transmissible under
natural conditions from animals to
– Bacterial: Leptospira, plaguefrom Rat.
– Viral : Rabies from dog.
– Protozoa: Leishmaniasisfromdog.
– Helminths : Hydatiddiseasefromdog
– Tape worms: Cattle,Pig.
Reservoir in non-living
Some organisms are able to
survive and multiply in
such as soil and water
Clostridium that causes tetanus
and botulism can survive many
years in the soil
Hookworms deposit their eggs
into the soil
Water contaminated by human
or animal feces cause GI tract
disease (list includes bacteria,
3rd - The Portal of Exit
• Route of escape of the pathogen from
the reservoir-IA entersintosurrounding
4th –Mode of Transmission
Contact with soil
Inoculation into skin or mucosa
•Inf spread by direct contact of
skin-skin, skin mucosa, mucosa-
mucosa of same or other person
•by touching, kissing,,
bites, or sexual intercourse
•Direct & immediate transfer of IA
from reserviour –host (no intermediate
•So it introduces larger dose of IA
•No time interval of survival in
•Overcrowded place or where
place with lack of ventilation
• Direct projection of
al secretion by
mucosa or skin
Infectious agent excreted in faeces
& transmitted to the oral
portal of entry through
contaminated food, water, milk,
• Typhoid fever
• Hepatitis A, Hepatitis E
Urinary & Reproductive
Breaks in Protective Skin
(bite of arthropod)
6th - The Susceptible Host
• A person or an animal that afford lodgment
to an infectious agent under natural
•Accepts the pathogen
•The support of pathogen life & its
reproduction depend on the degree of the
Obligate host : the only host
Eg: Man in measles & typhoid
Primary /definitive host: in which
parasite attains maturity or passes
its sexual stage
Secondary or intermediate hosts:
the parasite is in a larval or asexual
Male / Female
THE TIME INTERVAL BETWEEN INVASION BY
AN INFECTIOUS AGENT ANDAPPEARANCE OF
THE FIRST SIGN OR SYMPTOM OF THE
DISEASE IN QUESTION
DOSE OF INOCULUM
SITE OF MULTIFICATION
RATE OF MULTIFICATION
HOST DEFENCE MECHANISM
50% 50%Probable exposure
Median incubation time
No of cases
1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 21 22
Period From Disease Initiation To
No of cases
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
INTERVAL OF TIME BETWEEN RECEIPT OF
INFECTION BY A HOST ANDMAXIMAL
INFECTIVITY OF THAT HOST
THE GAP IN TIME BETWEEN
THE ONSET OF THE
PRIMARY CASE AND THE
It is defined as the time during which
an infectious agent may be transferred
directly or indirectly from an infected
person to another person, from an
infected animal to man , or from an
infected person to an animal, including
Ate the food (exposed) Did not eat the food (not exposed)
Ill Well Total Attack
Ill Well Total Attack
10 3 13 76% 7 4 11 64%
Attack Rate = Ill / (Ill + Well) x 100 during a time period
Attack rate = (10/13) x 100 = 76%
( 7/11) x 100 = 64%
It is defined as the number of exposed
persons developing the disease within the
range of the incubation period, following
exposure to the primary case
Used to estimate to the spread of disease in a
family, household or other group environment.
Measures the infectivity of the agent and the
effects of prophylactic agents (e.g. vaccine)
Totalnumberof cases– initialcase(s)