This document provides an introduction to epidemiology. It defines epidemiology as the study of disease frequency, distribution, and determinants in populations in order to control health problems. The key concepts covered include the components of infectious disease epidemiology such as the natural history of disease, causal concepts, the epidemiologic triangle, the chain of infection, and levels of disease occurrence. It also discusses the history and scope of epidemiology and provides learning objectives.

1. INTRODUCTION TO
EPIDEMIOLOGY
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2. LEARNING OBJECTIVES
At the end of this topic, students are able to:
• Define Epidemiology
• Describe theories of disease causation
• Identify components of infectious disease
epidemiology
• Discuss levels of diseases occurrence
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3. DEFINITION
• The study of the frequency, distribution and
determinants of health-related states or events in
specified human populations and its application to
the control of health problems
Key words:
Frequency: quantifies occurrence of diseases and health
related events
Distribution: refers to how disease is distributed within
the population
• Person, time and place
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4. Defn…
Determinants: refers to the ‘why’ and ‘how’ of disease
occurrence
• Causes, risk factors, mechanisms
Health-related states / events:
• Physiological/psychological dysfunction of the body
• Vital events, individual behavior, Socio-economic.
productivity
Population: it focuses on populations than individuals.
Application: Design effective prevention and control)
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5. HISTORY OF EPIDEMIOLOGY
Hippocrates (460 B.C): disease are related to personal and environmental
factors.
John Graunt (1662): first to quantify patterns of morbidity and mortality.
Lind (1747): proved the cause of scurvy and its treatment with fresh fruit.
John Snow (1854): used a "natural experiment" to test his hypothesis
linking Cholera to drinking contaminated water.
Austin B. Hill (1937): application of statistics for establishing causation.
1950's-1970's: Major epidemiology successes in the area of – tobacco, BP
and stroke, CHD risk factors, endometrial cancer and
exogenous estrogens.
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6. BASIC EPIDEMIOLOGIC ASSUMPTIONS
1. Human diseases do not occur at random
• Some behavioral and environmental factors increase the
risk of acquiring a particular disease.
2. Human diseases have causal and preventive factors
• They can be identified through systematic investigation of
populations or group of individuals.
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7. SCOPE OF EPIDEMIOLOGY
• Describe the health status of the population.
• Establish natural history of disease.
• Provide understanding of what causes or sustains
disease in populations.
• Define standards and ranges for normal values of
biological and social measures.
• Guide health and healthcare policy and planning.
• Assist in the management and care of health and
disease in individuals.
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8. Scope
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9. Step Clinician Epidemiologist
Data base
History, physical
exam
Surveillance,
descriptive epid.
Assessment Differential dx Inference
Hypothesis
testing Diagnostic
studies
Analytical
epidemiology
Action Treatment
Community
intervention
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10. BRANCHES OF EPIDEMIOLOGY
1. Descriptive Epidemiology
• Defines the amount and distribution of health problems
in relation to person, place and time.
• It answers the questions who, where and when.
2. Analytic Epidemiology
• involves explicit comparison of groups of individuals to
identify determinants of health and diseases.
• It answers the questions why and how.
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11. INFECTIOUS DISEASE
EPIDEMIOLOGY
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12. A. NATURAL HISTORY OF DISEASE
Refers to the progression of a disease process in an
individual over time, in the absence of intervention.
Figure 2.1. Natural History of Disease
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13. Figure 2.2 Natural History of Tuberculosis
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14. B. CAUSAL CONCEPTS OF DISEASE
Cause of disease: is an event, condition, characteristic or
a combination of these factors which plays an important
role in producing the disease.
• The causes of disease can be classified in to two:
i. Primary causes(biological)--------
• These are the factors which are necessary for a disease
to occur, in whose absence the disease will not occur.
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15. ii. Risk factors (contributing, predisposing, or
aggravating factors).
• These are not the necessary causes of disease but they
are important for a disease to occur.
• Not all associations between exposure and disease
are causal.
• If disease does not develop without the factor being present,
then we term the causative factor "necessary".
• If the disease always results in the presence of a factor, then
we term the causative factor "sufficient"
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16. Quiz: Identify the necessary and sufficient factors
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17. • The etiology of a disease is the sum total of all the
factors (primary causes and risk factors) which
contribute to the occurrence of the disease.
• The epidemiologic triad is the traditional model of
infectious disease causation.
• It has three components: an external agent, a
susceptible host, and an environment that brings the
host and agent together.
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18. Figure. EPIDEMIOLOGIC TRIANGLE AND TRIAD (BALANCE BEAM)
Host factors Agent factors Environmental factors
• Age, sex, weight,
• Personal Behavior
• Genetic inheritance
• Virulence, Serotype
• Antibiotic resistance
• Cigarette-tar content
• Overcrowding
• Air pollution
• Animal contact
Agent
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19. C. CHAIN OF INFECTION
Infection implies entry and multiplication of the agent,
whether or not the process leads to disease.
• A model used to understand the infection process is
called the chain of infection.
• Each link must be present and in sequential order for
an infection to occur.
• Understanding the characteristics of each link provides
with methods to prevent the spread of infection.
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20. Components of Chain of Infection
1. Causative Agent
2. Reservoir host(motly wild animals)
3. Portal of exit
4. Mode of transmission
5. Portal of entry
6. Susceptible host.
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21. Figure 2.8. Chain of infection
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22. 1. Causative agent: is an infectious micro-organism-virus,
bacteria, parasite, or other microbe
2. Reservoir host: is the habitat in which an infectious agent
normally lives, grows, and multiplies.
• Reservoirs for infectious agents may be humans, animals,
plants or other inanimate objects.
• Agents with a human reservoir include measles, mumps,
and most respiratory pathogens.
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23. • Human reservoirs may be persons with symptomatic
illness, or carriers.
• A carrier is a person who does not have apparent
clinical disease, but potentially capable of transmitting
the infection to other people.
Asymptomatic carriers
Incubatory carriers
Convalescent carriers
Chronic carriers
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24. • Some diseases are transmitted to human beings from
animals. These diseases are called zoonosis.
e.g. Rabies, anthrax
3. Portal of exit: is the way the infectious agent leaves
the reservoir.
• Possible portals of exit include all body secretions and
discharges.
─ Mucus, saliva, tears, breast milk, blood,
─ Vaginal and cervical discharges,
─ Excretions (feces and urine).
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25. 4. Modes of Transmission:
The mechanism by which an infective agent is transferred
from a reservoir to a susceptible host.
I. Direct Transmission:
• Direct contact – STIs, Rabies
• Direct Projection – Common cold
• Transplacental – Syphilis, HIV/AIDS
II. Indirect Transmission:
• Vehicle-borne – Cholera
• Vector-borne – Malaria
• Airborne – TB
• Parenteral injections – HBV
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26. 5. Portal of entry: is the site where an infectious agent
enters a susceptible host.
• Nasal mucosa – common cold
• Conjunctiva – trachoma)
• Injury site – tetanus)
6. Susceptible human host: the final link in the infectious
process.
• The chain of infection may be interrupted if the agent does
not find a susceptible host.
• Host resistance at the community (population) level is called
herd immunity.
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27. E. LEVELS OF DISEASE OCCURRENCE
• Diseases occur in a community at different levels at a
particular point in time.
• Some diseases are usually present in a community at a
certain expected level.
I. Expected levels
a. Endemic: a persistent level of low to moderate occurrence
b. Hyper-endemic: a persistently high level of occurrence
c. Sporadic: occasional cases occurring at irregular intervals
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28. At times disease may occur in excess of what is expected
II. Excess of what is expected
a. Epidemic: occurrence of disease in excess of what is
expected in a limited period.
b. Outbreak: same as epidemic, often used by public health
officials for it is less provocative to the public.
c. Pandemic: an epidemic spread over several countries or
continents, affecting a large number of people.
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29. Number
of
cases
Figure. Levels of disease occurrence.
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