EPIDEMIOLOGY
Introduction and Disease
Transmission
Sue Lindsay, Ph.D., MSW, MPH
Division of Epidemiology and Biostatistics
Institute for Public Health
San Diego State University
Epidemiology
The study of patterns of health, disease, and injury in
human populations and the application of this
study to the control of health problems
ASPH Ten Epidemiology Competencies
Upon Graduation, a student with an MPH should be able to:
1. Identify key sources of data for epidemiologic purposes
2. Identify the principles and limitations of public health screening
programs
3. Describe a public health problem in terms of magnitude, person, time,
and place
4. Explain the importance of epidemiology for informing scientific, ethical,
economic, and political discussions of health issues
ASPH Ten Epidemiology Competencies
Upon Graduation, a student with an MPH should be able to:
5. Comprehend basic ethical and legal principles pertaining to the
collection, maintenance, use, and dissemination of epidemiologic data.
6. Apply the basic terminology and definitions of epidemiology.
7. Calculate basic epidemiology measures.
8. Communicate epidemiologic information to lay and professional
audiences.
ASPH Ten Epidemiology Competencies
Upon Graduation, a student with an MPH should be able to:
9. Draw appropriate inferences from epidemiologic data.
10. Evaluate the strengths and limitations of epidemiologic reports.
Underlying Assumptions
• Illness and disease are not randomly distributed
in human populations
• Each human being has characteristics that either
predispose toward illness, or protect from illness.
• Communities and neighborhoods also have characteristics
that either predispose toward, or protect from illness.
• These characteristics are identifiable and modifiable
As Medical Detectives
Epidemiologists Must Find:
• Who?
• What?
• Where?
• When?
• Why?
• How?
The Five Objectives of
Epidemiology
1. To identify the cause(s) of a disease and the risk
factors for that disease.
• How is the disease or condition transmitted or acquired?
Are there sub-groups of the population at high risk for the
disease?
2. To determine the extent of the disease found in a
community or population - surveillance
• What is the burden of the disease or condition?
The Five Objectives of
Epidemiology
3. To study the natural history and prognosis of the
disease
• Severity, lethality, duration, survivorship
4. To evaluate existing and new preventive and
therapeutic measures as well as modes of health
care delivery
• Does screening for disease impact outcome?
The Five Objectives of
Epidemiology
5. To provide the foundation for developing public
health policy and regulatory decisions
• How do environmental problems impact human health?
Epidemiologic Areas of Study
• Observational Epidemiology
• Natural Experiments
• Experimental Epidemiology
Historical Examples of
Epidemiology in Practice
The Story of Smallpox
• Major worldwide epidemic in the late 1700’s
• Known immunity from re-infection among
survivors
• “Variolation”: early attempts at control
were done by using infected smallpox pus
and tissue to “variolate” healthy people
The Story of Smallpox
• Dairy Maids - young women who milked
cows got mild disease known as “Cowpox”
• During smallpox outbreaks, dairy maids did
not develop smallpox
• Edward Jenner (born 1749), physician
practicing in England believed cowpox
could protect against smallpox
The Story of Smallpox
• 1778: Jenner decides to test his hypothesis
• Innoculates an 8 year old “volunteer” James
Phipps with cowpox material from a dairy maid
• Six weeks later Jenner exposes the boy to a
smallpox infection
• Smallpox did not infect the boy
The First Vaccination
The Story of Cholera
• Cholera was a major public health problem
in England in the mid-19th century
• First week of September 1854: 600 Deaths
among people living near Broad Street in
London
The Story of Cholera
• John Farr, Registrar General
• John Farr believed the disease was
transmitted by a cloud or “miasma”
clinging low to the earth
• He hypothesized that greater altitude would
be protective against cholera
Deaths from Cholera in 10,000
Inhabitants by Elevation Above Sea
Level, London 1848-1849
120
Feet above sea level
100
80
60
40
20
0
<20
2040
40- 60-80 8060
100
100- 340120 360
The Story of Cholera
• John Snow, physician to Queen Victoria
• Believed cholera was transmitted by contaminated
water
• Public water companies transported water supply
from polluted parts of the Thames River.
• The Lambeth Water Company moved their water
intake upriver towards non-polluted water
The Story of Cholera
• John Snow hypothesized that death rates would
be lower in households buying water from the
Lambeth Company
• 1854: Conducted a house to house survey
• Number of deaths/household
• Water company that supplied the household
Deaths From Cholera Per
10,000 Houses By Source of
Water Supply
Water
Supply
No of
Houses
Deaths
From
Cholera
Deaths per
10,000
Houses
Southwark
40,046
1,263
315
Lambeth
26,107
98
38
Other
256,423
1,422
56
Deaths From Cholera Per 10,000
Houses By Source of Water Supply
1952
Overweight and Obesity
1960-2000
The Epidemiologic Approach
• How does the epidemiologist identify public
health problems and design interventions?
• Frequency of health and disease
• Patterns of disease by age
• Patterns of disease by geography
• Patterns of disease by race/ethnicity
• Patterns of disease by gender
Frequency of Health and Disease
Ten Leading Causes of Death in the United States,
1900 and 1997
Patterns of Disease by Age
Life Expectancy At Birth and Age 65, By Race and Sex,
United States, 1900, 1950, 1996
Patterns of Disease by Geography
Adult/Adolescent AIDS Rates
per 100,000 White Population
Reported in 1999
6.1
1.3
7.0
1.2
7.7
3.1
3.0
2.5
1.9
2.4
2.8
7.6
17.7
3.0
3.6
13.1
6.3
7.0
7.7
5.8
20.6
5.5
3.5
3.6
6.5
5.8
6.2
3.9
13.1
*
6.9
6.7
8.7
5.5
7.9
10.1
19.6
21.1
Includes cases with unknown state of residence
3.5
15.4
RI
7.1
CT
NJ
DE
MD
DC
10.3
8.9
11.7
7.3
70.9
3.7
8.6
7.0
†
13.1
3.3
NH
MA
Rate per 100,000
<5
5-9.9
10+
* <5 cases
US rate =9.0
N=14,813
†
Adult/Adolescent AIDS Rates
per 100,000 Black Population
Reported in 1999
45.6
*
*
*
37.9
55.5
*
*
*
89.5
70.3
29.1
131.1
63.0 29.3
36.9
36.9
23.6
32.1
56.4
41.0
*
18.5
25.1
84.0
39.6
67.4
*
56.7
36.5
77.7
56.0
182.1
27.1
Includes cases with unknown state of residence
*
MA
RI
CT
NJ
DE
MD
DC
173.0
81.2
82.4
147.6
107.4
114.7
266.3
45.4
61.1
74.5
†
180.3
36.8
20.8
54.3
*
29.7
Rate per 100,000
<50
50-99
100+
* <5 cases
US rate =84.2
N=21,730
Adult/Adolescent AIDS Rates
per 100,000 Hispanic Population
Reported in 1999
15.3
*
15.7
*
31.8
14.9
*
*
*
32.3
14.8
108.6
19.3 12.2
16.3
16.2
25.4
23.4
20.0
29.2
*
7.8
6.6
*
32.3
57.9
25.8
Includes cases with unknown state of residence
19.3
20.6
*
7.1
22.6
P.R. 41.9
NH
60.7
MA 127.3
RI
60.0
CT
89.2
NJ
43.6
DE
42.7
MD
DC
23.5
112.2
25.6
32.5
17.2
†
124.5
15.9
17.6
24.7
21.9
*
*
43.2
Rate per 100,000
<20
20-49.9
50+
* <5 cases
US rate =34.6
N=8,967
†
Proportion of AIDS Cases, by Race/Ethnicity
and Year of Report,1985-1999, United States
70
White, not Hispanic
Percent of Cases
60
50
40
Black, not Hispanic
30
Hispanic
20
10
Asian/Pacific Islander
0
1985
1987
1989
1991
1993
Year of Report
American Indian/
Alaska Native
1995
1997
1999
AIDS Cases Reported in 1999 and Estimated
1999 Population, by Race/Ethnicity, United States
AIDS Cases
N=46,400*
Population
N=277,200,000
71%
32%
<1%
1%
47%
1%
4%
19%
13%
12%
White, not Hispanic
Black, not Hispanic
Hispanic
Asian/Pacific Islander
American Indian/
Alaska Native
*Includes 120 persons with unknown race/ethnicity
Noteworthy Examples of
Epidemiologic Investigations
• Tampons and Toxic Shock Syndrome
• Legonnaire’s Disease
• Low Level Ionizing Radiation and Leukemia
• Hormone replacement therapy and heart attack,
stroke, blood clots, breast cancer, reduced risk of
colorectal cancer
Noteworthy Examples of
Epidemiologic Investigations
• Passive Smoking
• Agent Orange
• Acquired Immune Deficiency Syndrome (AIDS)
• The Effect of DES on Off-Spring
• Severe Acute Respiratory Syndrome (SARS)
Asia 2003, 12+ countries, 8,098 sick, 774 died
• Avian Influenza (Bird Flu)
Disease Transmission
The Epidemiologic Triad
Host
Agent
Environment
Vector
An Example of The
Epidemiologic Triad
Host (Person)
Agent
(Bacterium)
Vector
(Mosquito)
Environment
(Contaminated
Water)
Factors Which Influence Health
and Disease in Humans
• Biological
• Physical
• Chemical and Environmental
• Genetics
• Nutrition
• Immunology
Host Characteristics
• Age
• Family Background
• Sex
• Previous Diseases
• Race
• Immune Status
• Occupation
• Genetic Profile
• Religion
• Customs
• Marital Status
Types of Agents
• Biologic
Bacteria, Virus
• Chemical
Poison, Alcohol,Smoke
• Physical
Trauma, Radiation,
Fire
• Nutrition
Diet, Low / Excess
Caloric Intake
Environmental Factors
• Temperature
• Water
• Humidity
• Milk
• Altitude
• Food
• Crowding
• Radiation
• Housing
• Air Pollution
• Neighborhoods
• Noise
• Violence
• Public health infrastructure
Modes of Disease
Transmission
• Direct:
Person to Person Contact
• Indirect:
• Vehicle borne
• Vector borne
• Single exposure
• Multiple exposures
• Continuous exposure
Body Surfaces as Sites of
Infection
•
Skin •
•
Mouth
Respiratory Tract
• Alimentary Tract
•Urinogenital Tract
The Iceberg Concept of
Infectious Diseases
Cell transformation/ dysfunction
Moderate/Severe Illness
Viral/Bacterial Transformation
Asymptomatic Infection
Exposure Without Cell Entry
Exposure Without Infection
Patterns of Disease
• Epidemic disease
• Endemic disease
• Pandemic disease
Epidemic Disease
• The occurrence of disease in a community
or region, clearly in excess of normal
expectations and derived from a common
or propagated source
Tuberculosis: Frequency Distribution
of Cases by Age in Minorities, United
States
450
Age
400
Number of Cases
350
300
250
200
150
100
50
0
0
10
20
30
40 50
60
70
80
Endemic Disease
• The habitual presence of a disease within a
given geographic area
Malaria
• WHO estimates 300-500 million existing
cases per year worldwide
• >90% of all cases occur in Sub-Saharan
Africa
• Malaria is endemic in 101 countries
reporting to the WHO
Pandemic: Worldwide Epidemic
• Three essential conditions for pandemics:
• A new infectious agent such that humans have
no natural immunity
• Agent must evolve to be capable of infecting
and killing humans efficiently
• Agent must succeed in being transferred from
human to human
World Health Organization (WHO)
Six Influenza Pandemic Phases
• Phase 1
• No viruses circulating among animals have infected
humans
• Phase 2
• An animal influenza virus has caused infection in
humans
• Phase 3
• Small clusters of disease in people. No human-tohuman transmission sufficient to sustain
community-level outbreaks.
World Health Organization (WHO)
Six Influenza Pandemic Phases
• Phase 4
• Human-to-human transmission able to cause
sustained community-level outbreaks.
• Phase 5
• Human-to-human transmission into at least two
countries in one WHO region. A global pandemic is
imminent.
• Phase 6
• Community level outbreaks in one other country in a
different WHO region. A global pandemic is
underway.
WHO Pandemic Phases
http://www.who.int/csr/disease/avian_influenza/phase/en/index.html
HIV/AIDS
• Worldwide epidemic
• No country is exempt
• WHO Estimates >42 million infected since
the pandemic onset
• Each day, >16,000 newly infected cases
worldwide
Herd Immunity
• The resistance of a group of people
to an attack by a disease because a
large proportionof the group members
are immune
Why Does Herd Immunity
Work?
• A large proportion of immune persons in
a population decreases the likelihood
that any one person with disease will
come into contact with susceptible
individuals
Characteristics of Herd
Immunity
• If a large percent of the
population is immune,
the entire population is
protected
• Infected persons are
unlikely to contact
susceptible persons
Herd Immunity in Public Health
• The success of immunization programs
depends on both immunization rates and
achieving herd immunity
• It may not be necessary to achieve 100%
immunization rates
Conditions for Herd Immunity
• Disease agent must be
restricted to single
host species
• Transmission must be
direct from one
member of host
species to another