Basic concepts in epidemiology

OUTLINE OF THE LECTURE

Models of disease causation
Epidemiologic triad concept
The natural

What is the disease ?

What is the definition of the disease ?

A satisfactory definition

Concept of disease causation

Supernatural theory of disease
Germ theory of disease
Ecological

Supernatural theory :

In the early past, the disease was thought mainly

Germ theory

Germ theory: Microbes (germs) were found to be the

Ecological theory

Around 463 BC, Hippocrates is the first epidemiologist who

Multifactorial theory

Pettenkoffer
Stated that agent, host and environmental factors will act

EPIDEMIOLOGICAL TRIANGLE
Changes in one of the elements of the triangle can

The traditional model of disease causation
3 components
an external agent
a susceptible host
an

Agent
Entity necessary to cause disease in a susceptible host
Examples
Biological (bacteria, virus,

Epidemiologic Triad - Agent

Infectivity – ability to invade a host
Pathogenicity –

Epidemiologic Triad Concepts

Infectivity, Pathogenicity , Virulence
all are dependent upon the condition

ENVIRONMENT

Environmental conditions ? major influence on health status of individuals in

The Natural History of Disease

The progress or course of disease in

Good health

Subclinical
changes

Recovery

Death

Clinical
disease

Natural History of Disease

Cholera
Dengue fever

The Natural History of Disease

1. Stage of susceptibility (before onset

Incubation Period

Interval between time of contact and/or entry of agent and

Dynamics of Disease Transmission

Transmissible
Infectious diseases
Genetic diseases

Non-Transmissible
Many chronic diseases, eg : diabetes

Mode of Transmission

Definition: mechanism through which infective agent move from reservoir

RESERVOIRS OF INFECTION

Definition:
habitats where infective agent can survive & (multiply).
Animals

Chain Of Transmission

Mode of Transmission

Direct:
From person-to-person
- Sexual contact
- Transdermal

Mode of Transmission
II. Indirect :
contaminated vehicles such as food, water,

Transmission of Disease

Fomites
Inanimate Objects
Tissues, towel, drinking glasses, needles
Droplet Transmission
Saliva and

Mode of Transmission

Person-to-person (respiratory, oral, genital, skin, body fluids)
Vector (insects)
Common vehicle

Iceberg Phenomena

VISIBLE
“TIP OF THE ICEBERG”

INVISIBLE

DEAD

SEVERE CASES

CASES (CLINICAL ILLNESS)

INVISIBLE CASES
(ASYMPTOMATIC

ICEBERG PHENOMENA

EXAMPLES:
Hepatitis B carriers
Cholera carriers
HIV/AIDS

HERD IMMUNITY

Definition: immunity or resistance of a HERD (population group or

Source: Timmreck, 1998

Source: Timmreck, 1998

HERD IMMUNITY

If the infectious agent cannot find a susceptible host to

Herd Immunity Threshold for Selected Vaccine-Preventable Diseases Modified from Am J Prev

SUMMARY

Quesetion

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