Important
words and concepts from Chapter 15, Black, 1999 (3/28/2003):
by Stephen T. Abedon (abedon.1@osu.edu)
for Micro 509
at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to access
text’s website Vocabulary
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(1) 
Chapter title: Epidemiology and Nosocomial Infections
(a)
"People
with infectious diseases are members of a population; they acquire infectious
diseases and transmit them within a population. Therefore, to further our
understanding of such diseases, we must consider their effects on populations,
including hospital populations."
(b)
[epidemiology and nosocomial
infections (Google Search)]
[index]
(a)
"The
study of factors and mechanisms involved in the frequency and spread of
diseases and other health-related problems within populations of humans, other
animals, or plants."
(b)
Typically,
if the cause of a disease is unknown, one approach to discovering its cause
employs epidemiology (a second, parallel approach might include more
traditional laboratory microbiological techniques)
(c)
[epidemiology and
"infectious disease" (Google Search)] [index]
(a)
The
(or a) cause, here particularly with
reference to infectious diseases
(b)
Etiologies
are causes of specific diseases, e.g., HIV is the etiology (or etiological
agent) of AIDS
(c)
[etiology and "infectious
disease" (Google Search)] [index]
(a)
The
incidence of a disease is the number of
new cases that occur within a given period of time
(b)
Typically
incidence is expressed in units of per year per 100,000 people
(c)
Measurement
of the incidence of disease gives a direct indication of how quickly a disease
is spreading within a population
(d)
[incidence and "infectious
disease" (Google Search)] [index]
(a)
Related
to but not identical to the concept of incidence is prevalence
(b)
Prevalence
is the absolute number of cases present within a population at a given instance
or over a period of time
(c)
The
prevalence differs from incidence particularly in including both old and new
cases
(d)
Prevalence
indicates to what extent a disease is impacting on a population since it
measures both the number of people affected and is affected by how long people
are affected
(e)
See Figure 15.1, Incidence
and prevalence of rates
(f)
[prevalence and
"infectious disease" (Google Search)] [index]
(6) 
Endemic
(a)
A
disease that is endemic is continually present within a population
(b)
Typically
the disease agent is present in too-low concentrations to affect a large number
of individuals (i.e., prevalence is low)
(c)
See Figure 15.2, The
incidence rate of chickenpox in the United States
(d)
[endemic infectious disease
(Google Search)]
[index]
(a)
An
epidemic refers to a higher-than-normal incidence of a disease within a
population
(b)
That
is, the number of cases of the disease is increasing
(c)
An
epidemic may or may not be caused by an endemic disease
(d)
[epidemic infectious disease
(Google Search)]
[index]
(a)
A
world-wide epidemic is called a pandemic
(b)
For
example, HIV infection is (or, at least, was) considered to be a pandemic as it
was increasing in incidence world-wide (and continues to do so in many parts of
the world)
(c)
[pandemic infectious disease,
pandemic disease, pandemic influenza,
pandemic HIV (Google Search)] [index]
(a)
A
disease that is not endemic and only randomly and unpredictably present in a
population is termed sporadic
(b)
See Figure 15.3, The
incidence of St. Louis encephalitis in the United States
(c)
See Figure 15.6, Incidence
rates of three different types of encephalitis
(d)
[sporadic infectious disease
(Google Search)] [index]
(a)
A
common-source outbreak is an epidemic that arises from contact with a
contaminated substance, such as a contaminated water source
(b)
The
typically defining feature of a common-source outbreak is a lack of passage of
the infectious agent from person to person, at least among the secondary
carriers
(c)
Typhoid
Mary, on the other hand, represented a common source for the outbreak of
typhoid fever she brought on by, though a carrier, insisting on handling and
distributing food to the public
(d)
[common-source outbreak,
campylobacter, ciguatera, gastroenteritis, hepatitis, legionnaire (Google Search)] [index]
(a)
In
contrast to a common-source outbreak, a propagated epidemic has numerous
infectious foci, i.e., infected people
(b)
[propagated epidemic
(Google Search)] [index]
(a)
The
index case is the first identified case of an epidemic disease
(b)
["index case" and
"infectious disease" (Google
Search)] [index]
(a)
"Sites
in which organisms can persist and maintain their ability to infect are
essential for new human infections to occur. Such sites are called reservoirs
of infection."
(b)
Reservoirs
of infection can be both animate (e.g., humans and other animals) and inanimate
(e.g., soil and water)
(c)
By
far the most important reservoir of human infections is our fellow humans
(d)
[reservoirs of infection
(Google Search)] [ProMED-mail
(reporting on emerging infectious diseases)] [index]
(a)
Human
reservoirs of infection that fail to show significant outward signs of
infection are termed carriers
(b)
Carriers
may also be termed chronic where a chronic carrier continues to serve as a
reservoir even after apparent recovery from a disease
(c)
[carriers of infection
(Google Search)] [index]
(15) 
Zoonoses
(a)
Zoonoses
are diseases with animal reservoirs of infection
(b)
Generally,
the more similar an animal is to us (i.e., the more closely related evolutionarily),
the more likely they will be able to serve as a reservoir for a human disease,
as well, of course, to being susceptible to human diseases (note that our pets
should also be included among the "us" serving as reservoirs of
diseases affecting wild animals)
(c)
Nevertheless,
rabies is our most important zoonosis due to its severity as well as its
prevalence in wild- animal populations, plus our potential for coming into
contact with the disease via our pets; the best way to contain the incidence of
rabies among humans is the vaccination of pets against rabies virus
(d)
Note
that the more animal species a disease is endemic among and the greater its
prevalence in the wild, the more difficult it is to eradicate the disease; our
few successes against human diseases are against ones that have had etiologies
that uniquely infect humans (e.g., smallpox, polio)
(e)
See Table 15.1, Selected
zoonoses (with emphasis on those that occur in pets)
(f)
[zoonoses (Google Search)] [infectious diseases
(author unknown)] [index]
(a)
For
a disease to successful infect us and cause disease, it must enter our body and
then find its way to a location that it is capable of adhering to and then
growing (alternatively, the pathogen-produced toxin, e.g., in the case of food
poisoning, must successfully enter our bodies)
(b)
The
route through which the pathogen (or its toxin) enters our bodies is called its
portal of entry
(c)
Pathogens
typically have well-defined portals of entry and are not infectious if they
enter a different portal of entry
(d)
Typically
we possess defences against successful entry via a given portal of entry
(e)
See Figure 15.9, Portals of
entry for human pathogens
(f)
Note
that portals of entry include the various normally present openings into the
body (e.g., mouth, ears, nose, eyes, urethra, etc.) as well as abnormal entries
(e.g., broken skin, insect bites, surgical wounds) as well as the bridge
between a mother and a fetus (the placenta, with pathogens usually passing from
the mother to the fetus rather than the other way around); some parasites are
even capable of passing through unbroken skin
(g)
For
organisms that can enter through more than one portal, the virulence of an
infection can vary with portal of infection (e.g., Yersinia pestis and bubonic versus pneumonic plague)
(h)
Often,
but not always, the characteristics of a disease give reasonably good clues as
to its portal of entry (e.g., respiratory infection) as well as its portal of
exit
(i)
[portals of entry (Google Search)] [index]
(a)
Pathogens
typically exit from animals in body fluids including saliva, respiratory
fluids, feces, semen, vaginal secretions, urine, and blood
(b)
As
with portals of entry, different pathogens typically have their own distinctive
portals of exit such that different diseases are spread through the contact
with different body fluids
(c)
See Figure15.10, Portals of
exit for human pathogens
(d)
[portals of exit (Google Search)] [index]
(a)
There
exist a variety of modes of transmission
·
Indirect contact transmission
(ii)
Vehicle
transmission
·
Airborne transmission (including dust particles)
(iii)
Vector transmission
(b)
See Figure 15.11, Modes of
disease transmission
(c)
["modes of
transmission" infectious disease (Google Search)]
[index]
(a)
Transmission
of a pathogen can be either horizontal or vertical
(b)
Horizontal
means transmission between individuals specifically who are not related as a
parent is to its offspring
(c)
[horizontal transmission
(Google Search)] [index]
(a)
Vertical
transmission occurs from parent to offspring, e.g., in utero, during passage
down the birth canal, or in breast milk
(b)
Typically
pathogens that have evolved for vertical transmission cannot be as virulent as
those that have evolved for horizontal transfer since the recipient of the
transfer must survive until reproductive maturity to pass on the pathogen
(c)
However,
note that not all (nor even most)
pathogens that may be transmitted vertically have evolved for that mode of
transmission (so, just as hybridomas and monoclonal antibodies typically are
not made by employing recombinant DNA technology, don't leave this discussion thinking that all pathogens that may be
passed from parent to offspring are harmless--most are instead just in the
wrong place at the wrong time so far as the poor baby is concerned)
(d)
[vertical transmission
(Google Search)] [index]
(a)
Contact
transmission requires either the direct or indirect contact with a reservoir of
infection
(b)
["contact
transmission" and disease (Google
Search)] [index]
(22) Direct contact transmission
(a)
Direct
contact transmission requires direct contact between individuals
(b)
[direct contact transmission
(Google Search)] [index]
(23) Direct fecal-oral
transmission
(a)
Direct
fecal-oral transmission is an example of direct contact transmission where
inappropriate hygiene (or kinky sexual practices) result in the direct contact
between an individual and another individual's feces
(b)
Fecal-oral
contact can also occur via indirect routes as discussed below
(c)
Fecal-oral
contact, direct or not, is how pathogens with oral portals of entry and fecal
portals of exit (many gastrointestinal diseases) get passed from individual to
individual
(d)
[direct "fecal-oral",
fecal-oral (Google Search)] [index]
(24) Indirect contact
transmission
(a)
Indirect
contact transmission does not involve
direct contact between individuals
(b)
Instead,
contact is made between a healthy individual and the disembodied body fluid of
a carrier (or diseased individual)
(c)
[indirect contact transmission
(Google Search)] [index]
(a)
Indirect
contact transmission typically occurs via fomites which are inanimate objects
upon which a pathogen has been deposited
(b)
Indirect
contact transmission occurs via contact between a healthy individual and a
pathogen-carrying fomite
(c)
E.g.,
bed sheets, eating utensils, etc., just about any inanimate object a person can
come into contact with can serve as a fomite, though some fomites serve as less
hostile environments for the pathogen than others
(d)
Note
that fomites are not vehicles of transmission and that vehicles
of transmission are not fomites
(e)
[fomites (Google Search)] [CDC hospital infections program: laundry] [index]
(a)
Also
among contact transmission is droplet transmission which are the droplets of
respiratory fluids that are projected out of one's mouth or nose when one
coughs, sneezes, or talks
(b)
Pathogens
with respiratory portals of exit often are transmitted by droplets
(c)
Note
that droplets have a finite life span and droplet transmission is considered to
have occurred only if the droplet is contacted within one meter of its ejection
from the ejecting individual
(d)
[droplet transmission
(Google Search)] [index]
(a)
A
vehicle is a substance that is normally brought into the body, i.e., water,
air, and food
(b)
Pathogens
hitch rides on (in) vehicles as a means of entering a body through a portal of
entry
(c)
Note
that fomites
are not vehicles of transmission and that vehicles of transmission are not
fomites
(d)
[vehicle transmission of
disease (Google Search)]
[index]
(a)
Many,
especially pathogens with anal portals of exit, may contaminate water and
thereby become transmitted via waterborne transmission (and would be described
as indirect fecal-oral transmission)
(b)
Note
that different pathogens display differences in their ability to survive in
aqueous environments
(c)
The
prevalence of coliform bacteria (as well as coliphages) is often employed as an
indicator of fecal contamination
(d)
[waterborne transmission of
disease (Google Search)]
[index]
(a)
Air,
especially dry air exposed to sunshine, is a hostile environment for many
pathogens