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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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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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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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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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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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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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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