Epidemiology- I
Mahmood ur Rahman
Definition of Epidemiology
Epidemiology is the study of the distribution
and determinants of diseases or states in
population groups and the application of
this study to the control of health problems
(Last JM ed. Dictionary of Epidemiology, Oxford University Press, 1995)
'Epidemiology is that field of medical
science which is concerned with the
relationship of various factors and
conditions which determine the
frequencies and distributions of an
infectious process, a disease, or a
physiologic
state
in
a
human
community'
(Lilienfeld 1978)
Historical Perspective
• Hippocrates - 5th century
 Association between external
environment and personal
characteristics and health
“Whoever wishes to investigate medicine properly should proceed thus:
in the first place consider the seasons of the year, and what effects each
of them produces. Then the winds, the hot and the cold, especially such
are as common to all countries, and then such as are peculiar to each
locality. In the same manner, when one comes into a city to which he is a
stranger, he should consider its situation, how it lies as to the winds and
the rising of the sun; for it influence is not the same whether it lies to the
north or the south, to the rising or to the setting sun. One should consider
most attentively the waters which the inhabitants use, whether they be
marshy and soft, or hard and running form elevated and rocky situations,
and then if saltish and unfit for cooking; and the ground, whether it be
naked and deficient in water, or wooded and well watered, and whether it
lies in a hallow, confined situation, or is elevated and cold; and the mode
in which the inhabitants live, and what are their pursuits, whether they are
fond of drinking and eating to excess, and given to indolence, or are fond
of exercise and labor.” (Hippocrates, “On airs, waters and places” Medical
Classics 3:19, 1938).
Historical Perspective
• John Graunt – 1662 (Hennekins and Buring 1987)
The Nature and Political Observations Made Upon the Bills
of Mortality
 Systematic statistical approach
• Analyzed births and deaths in London
– Excess of males born, higher mortality for males
– Infant mortality is very high
– Seasonal variation for mortality
 Importance of routinely collected information for study of
human illness
• William Farr - 1839
• Examined mortality and occupation and marital status
 Identified important issues in epidemiological investigations
• Use of comparison population, influence of multiple factors on
disease
Historical Perspective
• John Snow (1854) – Father of
modern epidemiology
• Established modern epidemiologic
methods
 Cholera epidemic in London
• Plotted geographical location of all cases – deaths from
cholera
From The Visual Display of Quantitative Data,
Edward R. Tufte
John Snow (cont)
 Went door to door, collecting information on
daily habits
• Suspected water supply as source of epidemic
– Broad street pump closed, epidemic stopped
 Mode of investigation – “shoe leather”
 Practical application of epidemiology – use
epidemiological investigation to impact a health
problem
How the Epidemiologist Works
• Studies origin and distribution of a health
problem
• Collection of data
• Constructs a logical chain of inferences to
explain the various factors in a society or
segment of society that cause a health
problem to exist
 Likened to a detective investigating the scene of a
crime looking for clues
• Starts with examination of sick person(s)
• Extends investigation to the setting where illness is
occurring
• Looks for common denominator that links all the
affected so that the cause of the problem can be
eliminated or controlled
Epidemiologic Analyses – Areas
of Study
Causal agents related to disease:
1. Biological agents – bacteria, viruses, insects
2. Nutritional agents – diet (fats, carbohydrates,
food nutrients)
3. Chemical agents – gases, toxic agents
4. Physical agents – climate, vegetation, chemical
pollutants (air, water, food)
5. Social agents – occupation, stress, social class,
lifestyle, location of residence
Epidemiologist studies:
Host characteristics:
•

Biological factors
•

Age, sex, degree of immunity, other physical
attributes that promote resistance or
susceptibility
Behavioral factors
•
Habits, culture, lifestyle
Social environment
•


Living conditions such as poverty, crowding
Norms, values and attitudes
•
Socially prescribed standards of living
–
–
Use of food and water, food handling practices
Household and personal hygiene
Eras of Epidemiology
1. Sanitary era – early 19th century
2. Infectious disease era – between late 19th
century and early 20th century
3. Chronic disease era – 2nd half of 20th
century
4. Eco-epidemiology era – 21st century
Comparing the job of a clinician
and the job of an epidemiologist
•
•
•
•
•
•
The clinician
Deals with patients
Takes a history
Conducts a physical
Makes a diagnosis
Proposes a treatment
Follows up the patient
•
•
•
•
•
•
The epidemiologist
Deals with populations
Frames the question
Investigates
Draws conclusions
Gives recommendations
Evaluates programmes
Person
Attack rate of acute hepatitis by age and
sex, Baripada, Orissa, India, 2004
Cases
Age
Sex
Population
0-4
1
1012
Attack rate
per 1000
0.1
5-9
11
21802
2
10-14
37
74004
5
15-44
416
51358
81
45+
73
56153
13
Male
341
102683
3.3
Female
197
101646
1.9
Uses of epidemiology
1. Examine causation
2. Study natural history
3. Description of the health status of
population
4. Determine the relative importance of
causes of illness, disability and death
5. Evaluation of interventions
6. Identify risk factors
1. Examine causation
Genetic
factors
Good health
Environmental factors
(Biological, chemical,
physical, psychological
factors)
Ill health
Life style
related factors
To study historical rise and fall
of disease in the population
• The diseases wax and wane (tuberculosis )
new ones appear (encephalitis, asbestos
poisoning, LSD psychosis) old ones are
eradicated (smallpox)
Number of countries with smallpox,
1967-1978
A Pertussis by 5-Year Age Groups
B. Pertussis by <1, 4-Year, Then 5-Year
Age Groups
Lung Cancer Rates in the United States,
1930–1999
Infant Mortality Rates for 2002, by
Race and Ethnicity of Mother
Place
Attack rate of acute hepatitis by zone
of residence, Orissa, India, 2004
Attack rate
0 - 0.9 / 1000
1 - 9.9 / 1000
10 -19.9 / 1000
20+ / 1000
Underground water supply
Pump from river bed
Common Source (Vehicle), Single
(Point) Exposure
Outbreak of Cholera, London, AugustSeptember , 1854
Common Source, Continued exposure :
Propogated source
Seasonal fluctuations :
• Malaria and JE are commoner during
immediate post monsoon season.
• Asthma shows highest incidence during
spring and autumn suggesting specific
environmental factors in its causation
Secular trend
Searching for causes and risk factors
• Theories in Primitive and Middle Ages
• Supernatural Causes e.g. being possessed by evil
spirits, wrath of gods, punishment for evil deeds
during previous births “bad air”
• Contagion Theory
• William Farr -Miasma Theory
• Germ Theory - Henle-Koch postulates
• Epidemiological Wheel Theory
• The Theory of “Necessary” and Sufficient”
cause
Role of the host, the agent and the
environment in the occurrence of disease
Biologic,
Chemical,
Physical (injury, trauma)
Social
Psychological
AGENT
VECTOR
Genotype
Nutrition
Immunity
Behaviour
HOST
Sanitation
Weather
Pollution
Socio-Cultural
Political
ENVIRONMENT
Causes of Tuberculosis
Syndrome identification
• Epidemiologists are called “lumpers and splitters”
• The differentiation of hepatitis A from hepatitis B
and the distinction between several varieties of
childhood leukemia.
• “lumping” include the identification of many
manifestations of tuberculosis.
• Patterson-kelly syndrome of association between
dysphagia and iron deficiency anaemia but when
association was tested by epidemiological studies ,
it was not found.
Individual risk and chances: Life tableThe ‘risks’ the average male in England
Measures of disease frequency
• Prevalence
 Number of cases of a disease in a defined
population at specified point of time
• Incidence
 Number of new cases, episodes or events
occurring over a defined period of time
Incidence rate
I=
Number of people who get
the disease or condition
in a specified time
X Factor
Total population at risk
Population at risk
Total population
All women
(age groups)
Population at risk
0-25
years
Men
Women
25-69
years
25-69
years
70+
years
Eg. Population at risk in a study of carcinoma of cervix
Prevalence
P=
Number of people with
the disease or condition
at a specified time
X Factor
Total population at risk
How to remember this: think
about a bathtub
Community diagnosis :
• The definition of indicators is a pre-requisite
Indicators - to estimate the burden of illness
and the strategies for control.
• The main health indicators are expressed in
terms of crude age-adjusted or age-specific
mortality rates (such as infant mortality
rates, mortality for children under 5, or
maternal mortality rates), disease-specific
morbidity rates, and life expectancy at birth.
3. Study natural history
Death
Good health
Sub-clinical
disease
Clinical
disease
Recovery
Natural history of disease timeline
4. Evaluation of interventions
Treatment,
Medical care
Good Health
Health promotion
Preventive measures
Public health services
Ill Health
Evaluation
• Evaluation is the process of determining,
as systematically and objectively as
possible, the relevance, effectiveness,
efficiency, and impact of activities with
respect to established goals.
• Effectiveness refers to the ability of a
program to produce the intended or
expected results in the field;
effectiveness differs from efficacy,
which is the ability to produce results
under ideal conditions.
• Efficiency refers to the ability of the
program to reproduce the intended
results with a minimum expenditure of
time and resources.
Descriptive Epidemiology
Collecting all data describing occurrence of disease
• Person
• Disease predominates in smokers
• Disease predominates immuno-compromised individuals
• Place
• Disease is associated with an arid climate
• Disease is associated with a tropical climate
• Time
• Disease associated with the date of company picnic
• A particular year
Disease surveillance
1. Frequency with which the
disease occurs
2. Definition of the disease
3. Size of the population from
which the cases develop
4. Completeness of the
reporting of the cases.
Types of epidemiology
Distribution
− Frequency of health events
− By person, time and place
Descriptive
epidemiology
Determinant
Analytic epidemiology
− Search for causes or risk factors
− Response to a study hypothesis
− Use various epidemiologic methods
Health, disease or injury
Disease-specific
epidemiology
Application
Applied epidemiology
Epidemiologic triad
HOST
DISEASE
AGENT
ENVIRONMENT
Factors Associated with Increased
Risk of Human Disease
HOST
(Intrinsic)
•Age
•Gender
•Ethnicity
•Religion
•Customs
•Occupation
•Heredity
•Marital status
•Family background
•Previous diseases
AGENTS
•Biological (bacteria,
etc.)
•Chemical (poison,
alcohol, smoke)
•Physical (auto,
radiation, fire)
•Nutritional (lack,
excess)
ENVIRONMENT
(Extrinsic)
•Temperature
•Humidity
•Altitude
•Crowding
•Housing
•Neighborhood
•Water
•Milk
•Food
•Radiation
•Air pollution
•Noise
EPIDEMIOLOGY
Scientific basis of public health
Measure of mortality and morbidity
Risk (Cumulative incidence) / Prevalence / Incidence
Rates
Ratio
Proportion
• Ratio is one number divided by another number
(numerator doesn’t have to be included in
denominator—and vice versa)
• Proportion is a ratio in which the numerator is
included in the denominator
 It has no dimension because the unit of the numerator
cancels out the unit of the denominator
• Rate is a ratio
 The numerator is the number of events
• The unit is event (i.e., number of cases)
 The denominator is the sum of follow-up time
contributed by the people at risk of the event
• The unit is time or, more accurately, person-time to account
for duration of time of follow-up