Study Guide
Introduction to Epidemiology
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University:
Michigan State University -
Course:
EPI 200| A Multi-disciplinary Approach to Problems in Global Public Health and Epidemiology Academic year:
2024
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85
Pages:
73
Author:
Nossardisd7p
Introduction
to
Epidemiology Lecture Contents….
1. Epidemiology defined.
2. The components of epidemiology
3. The basic tenets of epidemiology
4. Major Epidemiology cases
5. History of epidemiology
6. Theories of disease causation
7. Measuring disease occurrence
(Prevalence and Incidence) Definitions…
Epidemiology is a core science of public health.
Public health
The science & art of
Preventing disease,
prolonging life, and
promoting health & efficiency
through organized community effort
(Winslow, 1920)
3 Definitions
Health: A state of complete physical, mental and
social well-being and not merely the
absence of disease or infirmity (WHO,1948)
Disease: A physiological or psychological
dysfunction
Illness: A subjective state of not being well
Sickness: A state of social dysfunction Definitions
Epidemiology
The science of the mass phenomena of
infectious diseases or the natural history
of infectious diseases. (Frost 1927)
The science of infective diseases, their
prime causes, propagation and
prevention. (Stallbrass 1931.) Definitions…
Epidemiology
“The study of the frequency, distribution
and determinants of health-related states
or events in specified populations and the
application of the study to the control of
health problems”.
(J.M. Last 1988)
6 Epidemiology as a Science and a
Method
Epi = upon, among
Demos = people
Ology = science, study of
Epidemiology = the science or the study of
epidemics
It is the scientific method of disease
investigation − Typically, it involves the
disciplines of biostatistics and medicine. Epidemiology
In Epidemiology, we ask the
following questions related to the
health event:
What is the event? ( The problem).
What is the magnitude?
Where did it happen ?
When did it happen?
Who is affected?
Why did it happen?
8 Epidemiology
In Epidemiology, we ask the following questions
related to the health action:
What can be done to reduce this problem and
its consequences?
How can it be prevented in future?
What action should be taken by the
community? By whom should these activities
be carried out?
9 Components of the definition
Study: Systematic collection, analysis
and interpretation of data
Epidemiology involves collection,
analysis and interpretation of health
related data
Epidemiology is a science
10 Components of epidemiology
Disease frequency:
The core characteristics of epidemiology
are to measure the frequency of diseases,
disability or death in a specified
population.
It is always as a rate, ratio and proportion.
This falls in the domain of biostatistics,
which is a basic tool of epidemiology.
11 Components of epidemiology
Disease frequency:
E.g.. Prevalence rate,
Incidence rates, Death
rate etc.
These rates are essential
for comparing the disease
frequency in different
populations or sub groups
of the same population
12 Components…
Distribution: The study of the pattern of an
event by person, place and time.
Epidemiology studies distribution of diseases
among subgroups of the population, in certain
geographic areas, and also any increase or
decrease over time.
It answers the question who, where and when?
This is descriptive epidemiology.
An important outcome of this step is
formulation of etiological hypothesis
13 PERSON DISTRIBUTION
•In descriptive studies disease is further
characterized by defining the persons who
develop the disease by age, gender, ethnicity,
occupation, marital status, habits, social class &
other host factors.
•These host factors help us to understand the
natural history of disease. PLACE DISTRIBUTION
•Study of the geography of the disease
(geographical pathology) is one of the important
dimensions of epidemiology.
•With the geographical pathology we learn the
differences in disease patterns between two
geographical areas (e.g. international, national, or
urban/rural differences).
•These variations may be due to variations in
population density, social class, deficiencies in
health services, levels of sanitation, education &
environmental factors. TIME DISTRIBUTION
The pattern of a disease may be described by the time of
occurrence
Epidemiologists have identified three kinds of time trends
or fluctuations in disease occurrence:
1. Short term fluations: Single (one incubation period and
one peak)(e.g. food poisoning)
or multiple or continuous exposure (well of
contaminated water-cholera)
Minamata disease in Japan??
2. Periodic fluctuation:
Seasonal: GI infection in Summer
Cyclic: Influenza every 7-10 years..antigenic variations).
3. Long–term or Secular trend (CVD, lung cancer) EPIDEMIC CURVE
EXPOSURE
NUMBER
OF CASES
TIME Components…
Determinants:
Factors the presence/absence of which
affect the occurrence and level of an event
Epidemiology studies what determines or
influences health events:
✓ It answers the question: how and why?
✓ Epidemiology analyzes health events
18 Components…
Determinants:
This aspect of epidemiology is known as
“analytical epidemiology”.
Analytical strategies help in developing
scientifically sound health programmes,
interventions & policies.
19 Components…
Diseases & other health related events
Epidemiology is not only the study of
diseases.
The focus of Epidemiology is not only
patients’ health as individuals, but
anything that may affect their health and
well-being.
✓ It studies all health related conditions
✓ Epidemiology is a broad science
20 Components…
Human population
Epidemiology diagnoses and treats
communities/populations
✓ Clinical medicine diagnoses and treats
patients
✓ Epidemiology is a basic science of
public health
21 Components…
Application
Epidemiological studies have direct and
practical applications for prevention of
diseases & promotion of health
✓ Epidemiology is a science and practice
✓ Epidemiology is an applied science
Epidemiology provides the data essential to
the planning, implementation & evaluation
of services for the prevention, control &
treatment of disease.
22 Fatalities Associated with Farm Tractors
In 1982, the number of
farm tractor-associated
deaths in Georgia was
described in terms of
time, place, and person
by using records from an
existing surveillance
system.
23 Fatalities Associated with Farm Tractors (time)
.
24 Fatalities Associated with Farm Tractors (person)
Goodman RA, Smith JD, Sikes RK, et al. Fatalities associated with farm tractor injuries: an epidemiologic study. Public Health Rep
1985;100: 329–33.
25 Fatalities Associated with Farm Tractors (place)
26 Data Sources and Collection Methods
Source
Individual persons
Method
•
•
Questionnaire
Survey
Example
•
•
•
Environment
•
•
Samples from the
environment (river
water, soil)
Sensors for
environmental changes
•
•
Foodborne illness outbreak
CDC’s National Health and
Nutrition Examination Survey
Health data on U.S. residents
Collection of water from area
streams — check for chemical
pollutants
Air-quality ratings
Health care
providers
•
Notifications to health
department if cases of
certain diseases are
observed
•
Report cases of meningitis to
health department
Nonhealth–related
sources (financial,
legal)
•
•
Sales records
Court records
•
•
Cigarette sales
Intoxicated driver arrests
27 Epidemiology Purposes in Public Health
Practice
• Discover the agent, host, and environmental
factors that affect health
• Determine the relative importance of causes
of illness, disability, and death
• Identify those segments of the population that
have the greatest risk from specific causes of
ill health
• Evaluate the effectiveness of health programs
and services in improving population health Investigating an Outbreak
I Keep six honest serving-men:
(They taught me all I knew).
Their names are What and
Where and When And How and
Why and Who.
Rudyard Kipling (1865–1936)
Define what will be studied
Find out where the problem is
Who gets it , When it is occurring
Try to explain why the problem has such a distribution
Do specific studies to find out how the problem is occurring The Five Ws of Epidemiologic Studies Basic tenets of epidemiology
✓ The target of a study in epidemiology is
human Population: the most common
population in epidemiology is the
population in a given area or country at a
given time.
✓Since the structure of population varies at
each time such variations also have to be
taken in to consideration during data
analysis.
31 Basic tenets of epidemiology
✓ Diseases do not occur randomly.
✓ Conclusions are based on comparisons:
comparing the rates of disease frequency among
the exposed and unexposed is an important
epidemiological method.
✓Description of events by time, place and person.
Getting answer for when, where and who are
affected is very important in epidemiology to
formulate hypothesis about its causation. Other
important aspects are what, why and how of the
events.
32 Definition of Epidemic, Endemic,
and Pandemic
Endemic:
The habitual presence of a disease within a given
geographic area
May also refer to the usual prevalence of a given
disease within such an area
(APHA)
Epidemic or outbreak:
The occurrence of a disease in a community or region,
clearly in excess of normal expectancy (APHA)
• Pandemic:
A worldwide epidemic Legionnaire’s disease outbreak
American Legion
Convention,
Philadelphia,
Pennsylvania
July 21–24
Health care provider at a veterans’ hospital
in Philadelphia calls CDC to report cases
of severe respiratory illness among attendees
of the American Legion Convention
July 26–Aug 1
August 2
(Morning)
18 deaths
reported among
conventioneers
August 2
(Evening)
71 additional
cases reported
.
34 Legionnaire’s disease outbreak
• Members of the American Legion gathered for the annual American
Legion Convention held July 21 through 24, 1976, in Philadelphia.
• Soon after the convention began, a substantial number of attendees
were admitted to hospital emergency departments or were
examined in doctors’ offices with acute onset of fever, chills,
headache, malaise, dry cough, and muscle pain.
• More troublesome is that during July 26 to August 1, a total of 18
conventioneers died, reportedly from pneumonia.
• On the morning of August 2, a nurse at a veterans’ hospital in
Philadelphia called CDC to report cases of severe respiratory illness
among convention attendees.
• Subsequent conversations that day with public health officials
uncovered an additional 71 cases among persons who had attended
the convention.
• The goal was to find out why these conventioneers were becoming
ill and, in some cases, dying!!! Legionnaires’ Disease Cases, by Day
36 Legionnaires’ Disease Attack Rates
Hotel A
Age
(yrs)
Hotel B
Hotel C
Ill
Total
Percent
ill
Ill
Total
Percent
ill
Ill
Total
Percent
ill
39
3
44
6.8
3
116
2.6
6
160
3.7
40–49
9
160
5.6
11
232
4.7
20
392
5.1
50–59
27
320
8.4
25
523
4.8
52
843
6.2
60–69
12
108
11.1
19
207
9.1
31
315
9.8
70
11
54
20.4
5
76
6.5
16
130
12.3
0
2
0
0
7
0
0
9
0
62
688
9.0
63
1,161
5.4
125
1,849
6.8
Unknown
Total
37 Legionnaires’ Disease, by Age Group
Hotel A Residents
Time: July 21–24, 1976
Age (yrs)
Frequency
Unit size
Sick
Total
Percentage
39
3
44
6.8
40–49
9
160
5.6
50–59
27
320
8.4
60–69
12
108
11.1
70
11
54
20.4
Unknown
0
2
0
38 Legionnaires’ Disease Rate
Hotel A Residents
Time: July 21–24, 1976
Age (yrs)
Frequency
Unit
Sick
Total
Rate
Percentage
39
3
44
6.8
40–49
9
160
5.6
50–59
27
320
8.4
60–69
12
108
11.1
70
11
54
20.4
Unknown
0
2
0
39 Legionnaires’ Disease
• Five months after the first cases of Legionnaires’ disease
occurred, results of the case-control study indicated that
spending time in the lobby of Hotel A was a risk factor for
illness
• In January 1977, the Legionella bacterium was finally
identified and isolated and was found to be breeding in the
cooling tower of the hotel’s air-conditioning system; the
bacteria then spread through the building whenever the
system was engaged.
• Similar bacteria grew in warm waters in nature, such as hot
springs, and also had been identified in air-conditioning
cooling towers.
• The finding from this outbreak investigation lead to
development of new regulations worldwide for climate
control systems. London Smog Disaster, 1952
Air pollution causes respiratory illnesses and
death …
When fog and soot from coal burning created a
dense smog in Winter, 1952, in London, the smog
was around for five days from December 5–10,
1952
There was a substantial increase in mortality
The death rate in London in the previous week
was around 2,062
In the week of the smog, 4,703 died Epidemiology and Polio Vaccine
In April, 1955, Dr. Thomas Francis,
director of Poliomyelitis Vaccine
Evaluation Center at the University
of Michigan, announced that the two-year field trial of
the Salk vaccine against polio was up to 90% effective
“The results announced by Francis effectively marked
the beginning of the end of polio as the most lifethreatening and debilitating public health threat to the
children of the United States“. Scope of Epidemiology
Originally, Epidemiology was concerned
with investigation & management of
epidemics of communicable diseases
Lately, Epidemiology was extended to
endemic communicable diseases and noncommunicable diseases
Recently, Epidemiology can be applied to
all diseases and other health related events History of Epidemiology
Seven land marks in the history of
Epidemiology:
1) Hippocrates (460BC): Environment &
human behaviors affects health
2) John Graunt (1662): Quantified births,
deaths and diseases.
3) Lind (1747): Scurvy could be treated
with fresh fruit
45 History…
4) William Farr (1839): Established application
of vital statistics for the evaluation of health
problems.
5) John Snow (1854): tested a hypothesis on
the origin of an epidemic of cholera in London.
6) Alexander Louis (1872): Systematized
application of numerical thinking
(quantitative reasoning).
7) Bradford Hill (1937): Suggested criteria for
establishing causation.
46 History…
✓ Epidemiological thought emerged in 460
BC
✓ Epidemiology flourished as a discipline
in 1940s
47 John Snow (1813–1858)
An English physician and modern
-day father of epidemiology
•He used scientific methods to
identify the cause of the epidemic of cholera
in London in 1854
•He believed that it was the water pump on
Broad Street that was responsible for the
disease
The removal of the pump handle ended the
outbreak History of epidemiology
John Snow conducted the series of investigations in
London that later earned him the title father of field
epidemiology. Snow conducted his classical study in
1854 when an epidemic of cholera developed in the
golden square of London. During the time of
microscope development, snow conducted studies of
cholera outbreak both to discover the causes of
diseases and prevent its recurrences.
During that time Farr and Snow had major disagreement
about the cause of cholera. Farr adhered to what was
the called miasmatic theory of diseases, according to
this theory which was commonly held at a time
diseases was transmitted by a miasma or cloud that
clung low on the earth surface.
49 History of epidemiology
However Snow did not agree, he believed that
cholera is transmitted through contaminated
water. He began his investigation by determining
where in this area persons with cholera lived and
worked. He then used this information to map for
distribution of diseases.
Snow believed that water was the source of
infection for cholera. He marked the location and
searched the relationship between cases and
water sources.
He found that cholera was transmitted though
contaminated water. This was a major
achievement in epidemiology.
51 Theories of Disease Causation
What causes a disease?
Nineteenth century theories
1. Contagion theory
2. Supernatural theory
3. Personal behavior theory
4. Miasma theory
57 Theories …..
Twentieth century theories
1. Germ theory
2. Lifestyle theory
3. Environmental theory
4. Multi-causal theory
58 How do we measure diseases
occurrence?
Four quantitative descriptors:
•Numbers
•Ratios
•Proportions
•Rates Numbers: Use of actual number of events
e.g 100 cases of TB in community A
Ratios: Quantifies the magnitude of one
occurrence X, in relation to another event Y as
X/Y ( quantities X and Y are random).
The numerator is not a part of the denominator.
e.g Ratio of TB cases in community A to B is
1:10
60 Proportions: a ratio in which the numerator
is included in the denominator
e.g proportion of TB cases in community A is 10%
Rates: a proportion with time element
It measure the occurrence of an event
overtime (usually with a multiplier: 1000 or
100,000)
e.g US measles cases in 2000/US population in 2000
61 Types of rates
1. Crude rates: Apply to the total population in
a given area
2. Specific rates: Apply to specific subgroups in
the population (age, sex etc) or specific
diseases
3. Standardized rates: used to permit
comparisons of rates in populations which
differ in structure (e.g age structure), we can
calculate age or sex- standardized rates.
Two methods of standardization: Direct, indirect
62 Measurement of Disease Occurrence
Morbidity rates
Morbidity rates are rates that are used to quantify
the magnitude/frequency of diseases.
1. Incidence rates (Cumulative incidence, incidence
density).
2. Prevalence (Period prevalence, point prevalence).
63 Incidence rate
✓ The proportion of a population that develops
a disease overtime
✓ The risk/probability of an individual
developing a disease overtime
✓ The rapidity with which new cases of a
disease develop overtime
✓ The proportion of unaffected individuals who
on average will contract the disease overtime
65 Cumulative incidence
Cumulative =
Incidence
Number of new cases of a
disease during a specified period
Population at risk in the same
Period of time
66 Practical challenges in measuring
incidence rate
1.
Identification of population at risk
Population at risk constitutes all those free
of the disease and susceptible to it
2. Population is not static/it fluctuates/as a
result of births, deaths and migration
3. People are at risk only until they get the
disease and then no more at risk
67 Prevalence rate
It measures the proportion of a
population with a disease during a
specified period or at a point in time.
There is Point prevalence and Period
prevalence.
68 Point prevalence rate
Measures the proportion of a population
with a disease at a point in time
Point prevalence rate=All persons with a
disease at a point in time/Total
population
It is not a rate, but a true proportion.
69 Prevalence measures all of the current cases of the
disease in the community.
✓ It depends on the duration of the disease
process
✓ It depends on the incidence of the disease
❖ It can be used to determine the health care
needs of a community.
P=IXD
where P = Prevalence rate, I = Incidence rate, D =
Duration of the disease.
✓ Prevalence rate is equal to Incidence rate in case
of diseases with short duration or highly fatal
such as Rabies.
70 Relationship between
prevalence & incidence rates
An increase in prevalence rate
may not necessarily be due to an
increase in incidence rate, it
could be due to an increase in
average duration of a disease due
to decrease in death and/or
recovery rates.
71 Calculations …
A survey of respiratory disease was conducted and the
results are presented in the table below.
Calculate the prevalence of chronic bronchitis in each
age group and in the total group.
Prevalence of chronic bronchitis, by age, in a sample of 2383
employed men: , 1981.
Age (years)
Number
Surveyed
Frequency Prevalence (%)
45-49
496
18
3.6
50-54
672
18
2.7
55-59
1215
18
1.5
Total
2383
54
2.3
2 = 0.983, p = 0.612
Prevalence = 54 / 2383 = 0.0226 x 100% = 2.3%
= 0.0226x 1000= 22.6 cases/1000 pop. A study was conducted to examine the incidence of Carpal Tunnel
Syndrome (CTS) among computer operators in a certain corporation.
An initial survey was given to 12 administrative assistants. Two of
the 12 administrative assistants had symptoms and 10 did not reveal
signs or symptoms equivalent to CTS. The administrative assistants
who did not reveal signs or symptoms equivalent to CTS were then
recruited into a study and followed for 4 years. The findings are
listed below
3 of the 10 administrative assistants developed CTS during the 4 year
follow-up period
Subjects
1
1
1
2
1
4
Follow-up Time(yrs) CTS
1
yes
2.5
yes
3
yes
2
fired
1
transferred
4
no
Calculate Cumulative Incidence (per 1,000?)
Cumulative Incidence= 3/ 10 = 0.3 x 100% = 30%
= 0.3 x 1000 = 300 cases per 1,000 population
Introduction to Epidemiology
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