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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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MODES OF TRANSMISSION
A. Direct Transmission
1. Direct contact occurs through skin-to-skin
contact, kissing, and sexual intercourse.
o Direct contact also refers to contact
with soil or vegetation harboring
infectious organisms.
2. Droplet spread refers to spray with
relatively large, short-range aerosols
produced by sneezing, coughing, or even
talking.
o Droplet spread is classified as direct
because transmission is by direct spray
over a few feet, before the droplets fall
to the ground.
B. INDIRECT TRANSMISSION - refers to the transfer
of an infectious agent from a reservoir to a host
by suspended air particles, inanimate objects
(vehicles), or animate intermediaries (vectors).
1. Airborne transmission - occurs when
infectious agents are carried by dust or
droplet nuclei suspended in air.
o Airborne dust includes material that
has settled on surfaces and become
resuspendedby air currents as well as
infectious particles blown from the soil
by the wind.
2. Vehicle-borne - may indirectly transmit an
infectious agent include food, water,
biologic products (blood), and fomites
(inanimate objects such as handkerchiefs,
bedding, or surgical scalpels). A vehicle may
passively carry a pathogen as food or
water may carry hepatitis A virus.
3. Vector-borne such as mosquitoes, fleas,
and ticks may carry an infectious agent
through purely mechanical means or may
support growth or changes in the agent.
PORTAL OF ENTRY
The portal of entry refers to the manner in which
a pathogen enters a susceptible host.
The portal of entry must provide access to
tissues in which the pathogen can multiply or a
toxin can act.
HOST
The final link in the chain of infection is a
susceptible host.
Susceptibility of a host depends on genetic or
constitutional factors, specific immunity, and
nonspecific factors that affect an individual’s
ability to resist infection or to limit
pathogenicity.
An individual’s genetic makeup may either
increase or decrease susceptibility.
IMPLICATIONS FOR PUBLIC HEALTH
Knowledge of the portals of exit and entry and
modes of transmission provides a basis for
determining appropriate control measures.
In general, control measures are usually directed
against the segment in the infection chain that is
most susceptible to intervention, unless practical
issues dictate otherwise.

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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 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. RESERVOIR • 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) 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. • • 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. MODES OF TRANSMISSION A. B. Direct Transmission 1. Direct contact occurs through skin-to-skin contact, kissing, and sexual intercourse. o Direct contact also refers to contact with soil or vegetation harboring infectious organisms. 2. Droplet spread refers to spray with relatively large, short-range aerosols produced by sneezing, coughing, or even talking. o Droplet spread is classified as direct because transmission is by direct spray over a few feet, before the droplets fall to the ground. INDIRECT TRANSMISSION - refers to the transfer of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects (vehicles), or animate intermediaries (vectors). 1. Airborne transmission - occurs when infectious agents are carried by dust or droplet nuclei suspended in air. o Airborne dust includes material that has settled on surfaces and become resuspendedby air currents as well as infectious particles blown from the soil by the wind. 2. Vehicle-borne - may indirectly transmit an infectious agent include food, water, biologic products (blood), and fomites (inanimate objects such as handkerchiefs, bedding, or surgical scalpels). A vehicle may passively carry a pathogen — as food or water may carry hepatitis A virus. 3. Vector-borne such as mosquitoes, fleas, and ticks may carry an infectious agent through purely mechanical means or may support growth or changes in the agent. PORTAL OF ENTRY • • The portal of entry refers to the manner in which a pathogen enters a susceptible host. The portal of entry must provide access to tissues in which the pathogen can multiply or a toxin can act. HOST • • The final link in the chain of infection is a susceptible host. Susceptibility of a host depends on genetic or constitutional factors, specific immunity, and nonspecific factors that affect an individual’s • ability to resist infection or to limit pathogenicity. An individual’s genetic makeup may either increase or decrease susceptibility. IMPLICATIONS FOR PUBLIC HEALTH • • Knowledge of the portals of exit and entry and modes of transmission provides a basis for determining appropriate control measures. In general, control measures are usually directed against the segment in the infection chain that is most susceptible to intervention, unless practical issues dictate otherwise. Name: Description: ...
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