3. DEFINITION OF EPIDEMIOLOGY
Greek words
• Epi - meaning on or upon
• Demos - meaning people
• Logos - meaning the study of
In other words, the word epidemiology has its roots in the study of what
befalls a population.
“Epidemiology is the study of the distribution and
determinants of health-related states or events in
specified populations, and the application of this study to the
control of health problems”
4. DISTRIBUTION
Epidemiology is concerned with the frequency and pattern of
health events in a population
Frequency refers not only to the
number of health events such
as the number of cases of meningitis or
diabetes in a population, but also to the
relationship of that number to the
size of the population.
The resulting rate allows epidemiologists
to compare disease occurrence across
different populations.
Pattern refers to the occurrence
of health-related events by
time, place, and person.
5. Time patterns may be
annual, seasonal, weekly,
daily, hourly, weekday
versus weekend, or any
other breakdown of time
that may influence
disease or injury
occurrence.
Place patterns include
geographic variation,
urban/rural differences,
and location of work sites
or schools.
Personal characteristics
include demographic
factors which may be
related to risk of illness,
injury, or disability such as
age, sex, marital status,
and socioeconomic status,
as well as behaviors and
environmental exposures.
6. DETERMINANTS
• The causes and other factors that influence the
occurrence of disease and other health-related events.
• Any factor, whether event, characteristic, or other
definable entity, that brings about a change in a health
condition or other defined characteristic.
HOW???
Examples: Covid-19 (Health-related/event/ Effect)Corona
Virus (Cause/Factor) + high number of virus +low immunity
7. HEALTH-RELATED STATES OR
EVENTS
Anything that affects the well-being of a population.
Many epidemiologists still use the term “disease”.
The epidemiologist is concerned about the collective health of
the people in a community or population.
Specified populations
8. APPLICATION
The epidemiologist uses the scientific methods of descriptive
and analytic epidemiology as well as experience,
epidemiologic judgment, and understanding of local
conditions in “diagnosing” the health of a community and
proposing appropriate, practical, and acceptable public
health interventions to control and prevent disease in the
community.
9. FATHER OF FIELD EPIDEMIOLOGY
In the mid-1800s, an anesthesiologist named John Snow was
conducting a series of investigations in London
Snow conducted studies of cholera outbreaks both to
discover the cause of disease and to prevent its recurrence.
Disease/ Health Event Cholera
Factor/ Cause V.cholera (contaminated food/water)
10.
11. APPLICATION OF EPIDEMIOLOGY
1. Assessing the community’s health
2. Making individual decisions
3. Completing the clinical picture of diseases
4. Searching for causes
5. Estimating risk
6. Evaluating prevention/intervention programs
7. Investigating epidemics/diseases of unknown etiology
12. THE EPIDEMIOLOGIC
APPROACH
1. Case Definition
2. Numbers and Rates
3. Descriptive Epidemiology
• Organize and summarize data according to time, place, and
person.
• These three characteristics are sometimes called the
epidemiologic variables.
13.
14. THE EPIDEMIOLOGIC
APPROACH
4. Analytic Epidemiology
• Analytic epidemiology is concerned with the search for
causes and effects, or the why and the how
• Use analytic epidemiology to quantify the association
between exposures and outcomes and to test hypotheses
about causal relationships
15. CAUSATION OF DISEASE
The epidemiologic Triad
Host
Agent Environment
Cholera
Host- Human being
Env- Living area/ Water
Agent – V. Cholera
16. CAUSATION OF DISEASE
Agent factors
• Agent originally referred to an infectious microorganism –
virus, bacteruim, parasites or other microbe
• Generally, these agent must be present for disease to occur
• They are necessary but not always sufficient to cause
disease
17. CAUSATION OF DISEASE
Host factors (Intrinsic factors)
• Intrinsic factors influence an individual’s exposure, susceptibility
or response to a causative agent
• Age, race, gender, socio-economic status and behavior
(smoking, drug abuse, lifestyle, sexual practice, contraception,
eating habits) are just some of many host factors which affect a
person’s likelihood exposure.
• Age, genetic composition, nutritional and immunologic status,
anatomic structure, presence of disease or medication and
physiological makeup are some of the host factors which affect
person’s susceptibility and response to an agent
18. CAUSATION OF DISEASE
Environmental factors (Extrinsic Factors)
• Extrinsic factors affect the agent and the opportunity for
exposure
• Generally, environmental factors include:
1. Physical factors – geology, climate, physical
surrounding (nursing home, hospital)
2. Biological factors – insect that transmit the
agents
3. Socioeconomic factors – crowding, sanitation,
availability of health service
Dengue
Agent-Virus
Aedes- Mosquito
Host-Human
19. Host
Agent Environment
-Host genetic
-Stage of infection
-Antiretroviral therapy
-Reproductive tract infections
-Cervical ectopy
-male circumcision
-Contraception
-Menstruation & pregnancy
-HIV subtype
-Phenotypic differences
-Genotypic differences
-Antiretroviral drug resistance
-Social norms
-Average rate of sex-partner change
-Local prevalence/probability of exposure
-Social & economic determinants or risk
behaviour
Agent, Host & Environment factors associated with the sexual transmission of
HIV in a community
22. Rothman’s causal pies: Conceptual scheme for the causes of
a hypothetical disease
A B
C
D
E
A B
F
G
H
A C
F
I
J
SUFFICIENT
CAUSE
I
SUFFICIENT
CAUSE
II
SUFFICIENT
CAUSE
III
They are necessary but not always
sufficient to cause disease
X smoking
Smoking
Smoking
23. HILL’S POSTULATES
1. Strength of Association – the stronger the association, the less likely the
relationship is due to chance or a confounding variable
2. Consistency of the Observed Association – has the association been
observed by different persons, in different places, circumstances, and times? (similar
to the replication of laboratory experiments)
3. Specificity – if an association is limited to specific persons, sites and types of
disease, and if there is no association between the exposure and other modes of
dying, then the relationship supports causation
4. Temporality – the exposure of interest must precede the outcome by a period of
time consistent with any proposed biologic mechanism
5. Biologic Gradient – there is a gradient of risk associated with the degree of
exposure (dose-response relationship)
24. HILL’S POSTULATES
6. Biologic Plausibility – there is a known or postulated mechanism by
which the exposure might reasonably alter the risk of developing the
disease
7. Coherence – the observed data should not conflict with known facts
about the natural history and biology of the disease
8. Experiment – the strongest support for causation may be obtained
through controlled experiments (clinical trials, intervention studies,
animal experiments)
9. Analogy – in some cases, it is fair to judge cause-effect relationships
by analogy – “With the effects of thalidomide and rubella before us, it
is fair to accept slighter but similar evidence with another drug or
another viral disease in pregnancy
