Epidemiology is the study of the distribution and determinants of health and disease in populations. It has three main aims: to describe disease distribution and frequency, identify risk factors, and provide data to plan, implement, and evaluate disease prevention and control services. Descriptive epidemiology involves observing disease distribution by time, place, and person to measure disease frequency and form hypotheses about potential causes. Key aspects include defining the population and disease, systematically collecting data on time trends, geographic variations, and characteristics of individuals with the disease. This allows comparison to other populations and formulation of testable hypotheses about disease etiology.

1. Epidemiology
• Epi = Among;
• Demos = People;
• Logos = Study
“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”.
John M. Last - 1988

2. Aims of Epidemiology
• According to International Epidemiological Association (IEA),
epidemiology has 3 main aims:
a. To describe the distribution and magnitude of health and
disease problems in human population.
b. To identify aetiological factors (risk factors) in the
pathogenesis of disease; and
c. To provide the data essential to the planning,
implementation and evaluation of services for the
prevention, control and treatment of disease and to the
setting up of priorities among those services.
Historical Development of Epidemiology

3. 3 components of Epidemiology
1. Disease
Frequency
Epidemiology is
measurement of
frequency of disease,
disability or death,
and summarizing this
information in the
form of rates, ratio
and proportion (eg.-
prevalence rate,
incidence rate, death
rate etc.)
2. Distribution of Disease
Distribution of disease occurs
in patterns in a
community and that the
patterns may lead to
generation of hypothesis
about causative factors.
An important function of
epidemiology is to study
these distribution
patterns in the various
subgroups of the
population by time, place
and person
3. Determinants of
Disease
It is derived from first two
components, since
knowledge of
frequency and
distribution of
diseases is necessary
to test an
epidemiology. In this
process we study the
factors which affect
the disease frequency
and distribution. If
the determinants of
disease are
understood, its
control become easy.

4. Uses of Epidemiology
• To determine the three possible sets of disease factors; agent, host
and environment.
• To discover the etiology, or causative origin of a disease whose exact
cause is not yet known. For eg; London Cholera epidemic of 1849-54,
John Snow, established that cholera was caused by polluted water and
not by “bad air”.
• To study the occurrence of disease in a population for the purpose of
community diagnosis and prognosis.
• To investigate the epidemic of diseases in which the causative agents
are known, but the means of transmission are unknown.
• It helps for prevention, control and treatment of the community
health problems based on priorities.
• It is useful for estimating resource needs.
• To measure the risk thus providing data regarding the magnitude of
disease load and types of disease in the community in terms of
morbidity and mortality rate/s.

5. Cont.
• It is helpful for estimating the risk of
developing a disease by an individual.
• It also helps in implementation and evaluation
of health services such as medicine
production.
• Epidemiology is a useful administrative tool in
the implementation of a preventive medicine
program.

6. Types of epidemiological study
1. Observational study
a. Descriptive studies
b. Analytical studies
(i) Ecological or Correlational, with populations as unit
of study
(ii) Cross-sectional or Prevalence, with individuals
as unit of study
(iii) Case-control or Case-reference, with individuals
as unit of study
(iv) Cohort or Follow-up, with individuals as unit of
study

7. 2. Experimental studies Intervention studies
a. Randomized controlled trials or clinical trials
with patients as unit of study
b. Field trials with healthy people as unit of study
c. Community trials or Community intervention
studies with communities studies as unit of
study

8. Descriptive study
• The first phase of an epidemiological investigation
• Observing the distribution of disease or health-
related characteristics in human populations
• Identifying the characteristics with which the
disease in question seems to be associated
a. When is the disease occurring ?-time distribution
b. Where is it occurring? - place distribution
c. Who is getting the disease? - person distribution

9. Uses of descriptive epidemiology
a. provide data regarding the magnitude of the
disease load and types of disease problems in
the community in terms of morbidity and
mortality rate;
b. clues to disease aetiology, and help in the
formulation of an aetiological hypothesis;
c. provide background data for planning,
organizing and evaluating preventive and
curative services;
d. It helps in doing research;
e. describing variations in disease occurrence by
time, place and person

10. Procedure or process of descriptive epidemiology
1. Defining the population to be studied
• Population composition such as age, sex,
occupation, cultural characters
• Can be whole population or sample from it
• Can be specifically selected group such as school
children, hospital patient, small communities
• Provide denominator for calculating rates to
measure frequency of disease

11. 2. Defining the disease under study
• "operational definition", i.e., a definition by
which the disease or condition can be
identified and measured in the defined
population with a degree of accuracy.
• Clear criteria to measure disease in population
• Example, the presence of enlarged, red tonsils
with white exudates, which on throat swab
culture grow predominantly S. pyogenes.

12. 3. Describing the disease
• Occurrence and distribution of disease by time,
person and place
• Systematic collection and analysis of data
Time place person
Year, season Climatic zones Age Birth order
Month, week Country, region Sex Family size
Day, hour of onset Urban/rural
Local community
Marital state Height
Weight
Duration Towns, cities,
institutions
Occupation, social
status, Education
Blood pressure
Blood cholesterol
personalhabits

13. Time distribution
i. Short term fluctuations- occurrence of a disease is
an epidemic
ii. Period fluctuations
a. Seasonal trend: upper respiratory infections, malaria
b. Cyclic trend: disease occur in cycles spread over
short periods of time- days, weeks, months or years
e.g measles in pre vaccination era 2-3 years
iii. Long term or secular trends: consistent tendency to
change in a particular direction e.g coronary heart
disease, lung cancer

14. Place distribution
1. International variations: chronic disease in
developed countries and infectious disease in
developing country
2. National variations: based on geographical
locations and distribution
3. Rural- urban variations
4. Local distributions: boundaries and patterns
of disease distribution in and out cities

15. Person distribution
a. Age: disease according to age e.g measles in
childhood
b. Sex : sex specific morbidity and mortality rates
c. Ethnicity: disease occurrence according to racial
and ethnic origin
d. Marital status: risk factor for some disease and
conditions
e. Occupation: expose to particular types of risk
f. Social class: disease distribution according to
upper and lower class

16. 4. Measurement of disease
• measure disease load in population
• Measured in terms of mortality, morbidity and
disability
• Morbidity- incidence and prevalence
• Incidence from longitudinal study and prevalence
from cross sectional study

17. 5. Comparing with known indices
• To make comparisons between different
populations and sub groups and ask questions
• Possible to arrive at clues to disease aetiology

18. 6. Formulation of hypothesis
• Supposition or prediction arrived at from
observation
• Can be accepted or rejected using the techniques of
analytical epidemiology
• Hypothesis should specify:
The population
The specific cause being considered
The expected outcome- the disease
The dose-response relationship
The time- response relationship
"The smoking of 30-40 cigarettes per day causes lung
cancer in 10 per cent of smokers after 20 years of
exposure"