INTRODUCTION TO

EPIDEMIOLOGY
S H Nzala
SOM, UNZA
OBJECTIVES
1) Describe the origin of epidemiology
2) Define the term epidemiology
3) Outline the scope, uses and objectives of
epidemiology
4) Give the broad classification of epidemiology
 Introduction to Epidemiology

• Health
A state of complete physical, mental, and social well-being and
not merely the absence of disease or infirmity - World Health
Organization, 1948
• Public Health
Is an effort organized by society to protect, promote, and restore
the health of the population
Disciplines of Medicine
Basic sciences: anatomy, physiology, pathology etc.
Clinical sciences: paediatrics, obstetrics etc
- concerned with care for individual patient
Public health: community replaces the individual
 Background of Epidemiology

• Epidemiology draws upon the medical, biological, and behavioral

sciences (anthropology, psychology, sociology, and education), as well
as statistics, demographics health and medical care services and
computer sciences.
• We can study health and disease by
-Observing effects on individuals
•Laboratory investigation of experimental animals
•Measuring the distribution of health problems in the
population
Background
• We can study health and disease by
Observing effects on individuals
Laboratory investigation of experimental animals
Measuring the distribution of health problems in the
population
Origin
“Epidemiology” from Greek :
Epi = upon; Demos = people
‘epidemic’ = “upon the people” , Logos = study
Epidemiologists first concern was to investigate, control
and prevent epidemics
Two basic assumptions about disease:
• Disease does not occur at random
• Disease has causal and preventive factors
 Definition

• Study of the distribution and determinants of health-

related states and events in specified populations and the
application of this study to the control of health problems.
• Analytical tool for assessing the effectiveness of medical
intervention and health care delivery
• Basic science of public health that focuses on the
population at large.
 Epidemiology Definition (cont’d)
• Epidemiology
Disease
• deviation from physical, mental or
emotional health expands to include
conditions such as injuries, birth defects,
health outcome etc.
Population
• group of people often geographically
defined
 Epidemiology Definition (cont’d)
• Epidemiology
Distribution
• characterizing the distribution of health status in terms of age,
gender, race etc.
Determinants
• any factor that brings about a change in a health condition or
other defined outcomes.
 Meaning of the definition
• Study - includes: surveillance, observation, hypothesis testing,
analytic research and experiments.
• Distribution - refers to analysis of: times, persons, places and
classes of people affected.
• Determinants - include factors that influence health: biological,
chemical, physical, social, cultural, economic, genetic and
behavioural.
 Meaning of the definition
• Health-related states and events - refer to: diseases, causes of
death, behaviours such as use of tobacco, positive health
states, reactions to preventive regimes and provision and use
of health services.
• Specified populations - include those with identifiable
characteristics, such as occupational groups
• Application to prevention and control - the aims of public
health—to promote, protect, and restore health.
Determinants of disease
• Determinants of disease occurrence includes both
causes and factors that influence the risk of disease
• Disease is as a result of the epidemiologic triad
(Agent, Host and Environment)
• Infection occurs only when the AGENT is encountered
by a susceptible Host in an ENVIRONMENT that is
favourable
Two basic assumptions about disease:

• Disease does not occur at random

• Disease has causal and preventive
factors
• It results from an interaction of the
host (e.g. a person), the agent (e.g. a
bacterium) and the environment (e.g.
contaminated water supply)
 Epidemiologic Triad

Disease is the result of

forces within a
dynamic system
consisting of:
agent of infection
host
environment
Factors Influencing Disease Transmission

Agent Environment
• Weather
• Infectivity
• Housing
• Pathogenicity
• Geography
• Virulence
• Occupational
• Immunogenicity setting
• Antigenic stability • Age
• Air quality
• Survival • Sex
• Food
Host • Genotype
• Behaviour
• Nutritional (www)
status
Distribution of disease

• Frequencies of values or
categories of
measurement with
respect to time, place
and persons
 Frequency
• Involves measuring disease distribution
• Requires information
- count (quantification)
- size of population
- time period
• Requires mathematical calculation
- ratio
- proportion
- rate
Specialty disciplines:
Pharmacoepidemiology,
Clinical Epi,
Psycho-Social Epi,
Nutritional Epi,
Molecular Epi,
Genetic Epi,
Cancer Epi,
Environmental Epi,
Occupational Epi,
etc.,
 Scope
• Shift from infectious diseases initially to chronic diseases
of later life and now applied to other areas e.g.
-injuries
- adverse drug reactions
-mental illness
-family planning
-health services research etc.
 Scope

• Shift also to evaluation of certain exposures (chemicals,

ionising radiation)
• Evaluation of effects of new approaches to prevention
• Evaluation of treatment effectiveness etc
• Organization of health care
• Epidemiology not only concerned with epidemics but also
with inter-epidemic periods and with sporadic and endemic
occurrences of diseases
 Classification

Two broad categories:

a) Descriptive epidemiology: the study of the frequency
(amount) and distribution of health related state within a
population by person, place and time
b) Analytic epidemiology: more focused study of health
related problems or reasons for relatively high or low
frequency in specific groups.
Epidemiological questions
To describe the occurrence of disease fully, some broad
questions must be asked:
a) What health events are occurring?
b) Who is affected?
c) When do the cases occur?
d) Where do the cases occur?
Other questions
• Why is it occurring
• How can it be influenced
Uses of Epidemiology:
• Historical study - is community health getting better
or worse? We can only decide by comparing
experiences over time.
• Community diagnosis
• Working of health services
- availability, accessibility, utilization, effectiveness,
efficacy, efficiency
 Uses of Epidemiology:
• Individual risks and chances of getting disease
• Completing the clinical picture
- constructing a model
• Identification of syndromes “lumping and splitting”
• Search for causes
• Evaluation of presenting s/s of disease
- by analysing data in hospital charts
• Clinical decision making - involves use of decision trees
 Specific objectives of Epidemiology:
• to identify aetiology of disease and risk factors
• to determine extent of disease found in the community
(disease burden)
• to study the natural history and prognosis of disease
• to evaluate new preventive and therapeutic measures and new
modes of health care delivery
• to provide foundation for developing public policy and
regulatory decisions relating to environmental problems.
Epidemiology: a scientific tool

• To describe
• To understand
• To propose and test hypotheses
• To validate or challenge public
policy
DESCRIPTIVE
AND
ANALYTIC
EPIDEMIOLOGY
OBJECTIVES
• To describe the two broad categories of
epidemiology
• To lay the foundation for understanding study
designs
 Kinds of Epidemiology
Study of the occurrence
•Descriptive and distribution of disease
Further studies to determine the
•Analytic validity of a hypothesis
concerning the occurrence of
disease.
•Experimental Deliberate manipulation of the
cause is predictably followed
by an alteration in the effect
not due to chance
Descriptive vs. Analytic Epidemiology
Descriptive Analytic
• Used when little is  Used when insight
known about the about various aspects
disease of disease is available
• Rely on preexisting  Rely on development
data of new data
• Who, where, when  Why
• Illustrates  Evaluates the causality
potential
associations of associations
Both are important!
DESCRIPTIVE EPIDEMIOLOGY
PERSON, PLACE AND TIME
Key questions
• Why now?
• Why here?
• Why in this group?
Descriptive Epidemiology
• Study of the occurrence and distribution of
disease in terms of:
 Time
 Place
 Person
 What are the three categories of
descriptive epidemiologic clues?

□ Person: Who is getting sick?

□ Place: Where is the sickness occurring?
□ Time: When is the sickness occurring?

• PPT = person, place, time

Time
Secular

 Periodic

 Seasonal

 Epidemic
 DESCRIPTIVE
EPIDEMIOLOGY
In order to describe the occurrence of
disease fully, it is necessary to specify
Person, Place and Time
 Person, Place and Time

• Examining the distribution of disease in

a population in terms of descriptive
characteristics in order to:
1) Identify subgroups at highest risk
2) Find clues about possible causes
(hypothesis generating studies)
 DESCRIPTIVE EPIDEMIOLOGY
Descriptive epidemiology identifies
nonrandom variations in the distribution
of disease to enable an investigator to
generate testable hypotheses regarding
aetiology.
Population at Risk: What to ask for?

• Who gets the disease ? PERSON

• Where does the disease occur ? PLACE

• When does the disease occur ? TIME

Descriptive data answer question: who, when,

where about a disease or condition
 PERSON:
Three characteristics are of vital
importance: Age, Sex and Ethnic group
or race.
e.g. death rates fairly high in infancy,
decreasing to reach lowest point
between ages 5 - 14 and climbs
gradually up to age 40. Thereafter -
exponential.
 PERSON: Demographics (Age)
Describe the pattern of infection distribution by
the age groups in terms of population exposure
and susceptibility?
700

600

500

400

300

200

100

0
<1 (1-9) (10-19) (20-29) (30-39) (40-49) (50-59) (60-69) 70+

Age in years
 PERSON:
In general chronic conditions tend to
increase with age.
Frequency and Severity of disease may
also depend on age.
Age-sex distribution of some diseases
may reflect occupational exposure.
 PERSON: Sex
Death rates tend to be higher in
males than females but morbidity
rates are generally higher in females.
Depression: F>>M
 Person – Race/ethnicity

• Race/ethnicity
Major differences exist between culture,
behavior, health events and related activities
with subsequent impact on disease and
mortality patterns
It has to be considered in all analysis of
studies and its effect controlled for
 PERSON
Other variables would include:
a) social class (education, area of
residence, income, life style etc)
b) Blood Type- e.g ‘A’ - increased risk of
gastric cancer
‘O’ - increased risk of duodenal ulcer etc.
 PERSON: Other variables
c) Environmental exposures
chemicals (tobacco, asbestos), infectious
diseases (specific immunity)
d) Occupation,
e) Marital status- death rates vary from
lowest to highest in the order: married,
single, widowed, and divorced
 PERSON-Family variables:

a) Family size - associated with

social class,
b) Birth order - first borns at
higher risk of asthma,
schizophrenia, peptic ulcer and
pyloric stenosis
 PERSON-Family variables:

c) Maternal age - e.g congenital

abnormalities
d) Parental deprivation - e.g
psychiatric and psychosomatic
disorders, attempted suicide
 PLACE
May be Geographical, urban-rural
differences etc.
 TIME
• Occurrence usually expressed on a
monthly or annual basis
•Major types of variation over time
are: secular, cyclic, and short term
fluctuations.
 TIME
Secular trends: these are long term
variations- years or decades.
Cyclic trends : recurrent alterations in
the frequency of disease.
Cycles may be annual (seasonal) or
have other periodicity.
Descriptive Epidemiology
• Correlational studies

• Case reports

• Case series

• Cross sectional studies

Analytic Epidemiology

Determining the Etiology of Disease

 Types of Studies
• Cross-sectional – prevalence rates that may
suggest association (good for developing
theory, but no causal association)
• Retrospective (Case-control) – good for rare
diseases and initial etiologic studies
• Prospective (cohort, longitudinal, follow-up) –
yields incidence rates and estimates for risk.
Better for causal association.
• Experimental (intervention studies) – strongest
evidence for etiology
 Cross-Sectional Studies
• Single point in time (snapshot studies)
• Risk factors and disease measured at the same time
• Determines prevalence ratios
 Experimental Studies
• Uses an intervention in which the
investigator manipulates a factor and
measures the outcome
• Elements of a complete experiment
• Manipulation of data
• Use of a control group
• Ability to randomize subjects to
treatment groups
Classification of Epidemiology