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1 epidemiology and disease transmission

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EPIDEMIOLOGY AND DISEASE TRANSMISSION
EPIDEMIOLOGY
The word epidemiology comes from the Greek
words epi, meaning on or upon, demos, meaning
people, and logos, meaning the study of.
Epidemiology is 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
Study of interrelationship of factors in particular
areas in the field of health, and the application
of the results of such studies to influence such
factors to achieve a given outcome.
Study of ecology of health and disease, and the
application of knowledge gained to promote
health and control diseases.
CHAIN OF INFECTION
traditional epidemiologic triad model holds that
infectious diseases result from the interaction of
agent, host, and environment.
Transmission occurs when the agent leaves its
reservoir or host through a portal of exit, is
conveyed by some mode of transmission, and
enters through an appropriate portal of entry to
infect a susceptible host.
THE EPIDEMIOLOGICAL TRIAD OF CAUSAL FACTORS
AGENT
Any element, substance, or force whether living
or non-living, the presence or absence which can
initiate or perpetuate a disease process.
Could be physical and mechanical in nature
Chemicals
Exogenous (common poisons)
Endogenous (accumulation of toxic products)
CHARACTERISTICS OF AGENTS
Inherent Characteristics
o Physical features, biologic
requirements, chemical make-up,
viability, and resistance.
Characteristics directly related to man
o Pathogenicity measures the ability of
an agent when lodged in the body to
set-up a specific reaction, local or
general, clinical or sub-trial.
o Virulence- refers to the severity of the
reaction produced and is usually
measured in terms of fatality.
o Antigenicity - the ability to stimulate
the host to produce antibodies.
Characteristics in relation to the environment
o Refers to the reservoir and sources of
infection and modes of transmission.
RESERVOIR
- The habitat in which the agent normally lives,
grows, and multiplies.
- Includes humans, animals and the environment
- The reservoir may or may not be the source from
which an agent is transferred to a host.
HUMAN RESERVOIRS
o Many common infectious diseases
have human reservoirs.
o Ex: sexually transmitted diseases,
measles, mumps, streptococcal
infection, and many respiratory
pathogens.
ANIMAL RESERVOIR
o Many of these diseases are
transmitted from animal to animal,
with humans as incidental hosts.
o Zoonosis - refers to an infectious
disease that is transmissible under
natural conditions from vertebrate
animals to humans.
ENVIRONMENTAL RESERVOIR
o Plants, soil, and water in the
environment are also reservoirs for
some infectious agents.
o Many fungal agents, such as those that
cause histoplasmosis, live and multiply
in the soil.
PORTAL OF EXIT
Portal of exit is the path by which a pathogen
leaves its host.
The portal of exit usually corresponds to the site
where the pathogen is localized.

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CHARACTERISTICS OF AGENTS EPIDEMIOLOGY AND DISEASE TRANSMISSION EPIDEMIOLOGY • • • • The word epidemiology comes from the Greek words epi, meaning on or upon, demos, meaning people, and logos, meaning the study of. Epidemiology is 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 Study of interrelationship of factors in particular areas in the field of health, and the application of the results of such studies to influence such factors to achieve a given ou ...
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