Chapter 33
Epidemiology of Infectious Disease
Epidemiology
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science that evaluates occurrence, determinants,
distribution, and control of health and disease in a
defined human population
health
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disease
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condition in which organism (and all its parts) performs its
vital functions normally
impairment of the normal state of an organism or any of its
components that hinders the performance of vital functions
epidemiologist

one who practices epidemiology
Figure 33.1
Centers for Disease Control and
Prevention (CDC)
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located in Atlanta, GA
functions as national focus for

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developing and applying disease prevention and control
environmental health
health promotion and health education activities designed to
improve the health of the people
worldwide counterpart is the World Health Organization
(WHO) located in Geneva, Switzerland
Epidemiology Terminology

sporadic disease
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endemic disease
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occurs occasionally and at irregular intervals
maintains a relatively steady low-level frequency
at a moderately regular interval
hyperendemic diseases
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gradually increase in occurrence frequency above
endemic level but not to epidemic level
More Terms…

outbreak
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epidemic
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sudden, unexpected occurrence of disease
usually focal or in a limited segment of population
sudden increase in frequency above expected number
index case – first case in an epidemic
pandemic

increase in disease occurrence within large population over wide
region (usually worldwide)
Morbidity Rate
an incidence rate
 number of new cases in a specific time
period per unit of population

# new cases during a specific time
# individuals in population
Prevalence Rate
total number of individuals infected at any
one time
 depends both on incidence rate and
duration of illness

Mortality Rate

number of deaths from a disease per
number of cases of the disease
# deaths due to given disease
size of total population with disease
Correlation with a Single
Causative Agent
after recognition of infectious disease in a
population, outbreak correlated with
specific pathogen
 clinical microbiologists help in isolation
and identification of pathogen

Figure 33.4
Human Sources/Reservoirs
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carrier
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infected hosts who are potential sources of infection
for others
types of carriers
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active carrier has overt clinical case of disease
convalescent carrier has recovered but continues to harbor
large numbers of pathogen
healthy carrier harbors the pathogen but is not ill
incubatory carrier is incubating the pathogen in large
numbers but is not yet ill
Animal Reservoirs
transmission to human can be direct or
indirect
 vectors

 organisms
to another
that spread disease from one host
More Types of Carriers…

casual, acute, or transient carriers
 convalescent,
healthy, and incubatory carriers
that harbor pathogen for brief time

chronic carriers
 convalescent,
healthy, and incubatory carriers
that harbor pathogen for long time
How Was the Pathogen
Transmitted?
Four main routes:
 airborne
 contact
 vehicle
 vector-borne
Airborne Transmission
pathogen suspended in air and travels  1
meter
 droplet nuclei

small particles (1-4 m diameter)
 can remain airborne for long time
 can travel long distances
 usually propelled from respiratory tract of source
organisms by sneezing, coughing, or vocalization
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
dust particles also important route of airborne
transmission
Figure 33.9
Contact Transmission
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coming together or touching of source/reservoir and
host
direct contact (person-to-person)
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indirect contact
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physical interaction between source/reservoir and host
e.g., kissing, touching, and sexual contact
involves an intermediate (usually inanimate)
e.g., eating utensils, bedding
droplet spread

large particles (>5 m) that travel < 1 meter
Figure 33.8
Vehicle Transmission
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vehicles
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common vehicle transmission
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inanimate materials or objects involved in
pathogen transmission
single vehicle spreads pathogen to multiple hosts
e.g., water and food
fomites
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common vehicles such as surgical instruments,
bedding, and eating utensils
Vector-Borne Transmission
External (mechanical) transmission
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passive carriage of pathogen on body of vector
no growth of pathogen during transmission
Internal transmission
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carried within vector
harborage transmission – pathogen does not
undergo changes within vector
biologic transmission – pathogen undergoes
changes within vector
Virulence and the Mode of
Transmission
Evidence suggests correlation between mode of
transmission and degree of virulence
direct contact  less virulent
vector-borne  highly virulent in human host;
relatively benign in vector
 greater ability to survive outside host  more virulent
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e.g., Mycobacterium tuberculosis and Corynebacterium
diphtheriae can survive weeks to months outside human hosts
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Reasons for Increases in
Emerging and Reemerging
Infectious Diseases
crowding
 habitat disruption
 shift in distribution of nosocomial
pathogens
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More Reasons…
excessive or inappropriate use of
antimicrobial therapy
 medical practices that lead to
immunosuppression
 rapid global transportation systems

Control of Epidemics
Three types of control measures
 reduce
or eliminate source or reservoir of
infection
 break connection between source and
susceptible individual
 reduce number of susceptible individuals
Reduce or Eliminate Source or
Reservoir
quarantine and isolation of cases and
carriers
 destruction of animal reservoir
 treatment of sewage
 therapy that reduces or eliminates
infectivity of cases

Break Connection between
Source and Susceptible
Individuals
chlorination of water supplies
 pasteurization of milk
 supervision and inspection of food and
food handlers
 destruction of insect vectors with
pesticides

Reduce Number of Susceptible
Individuals
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raise level of herd immunity by
 passive
immunity following exposure
 active immunity for protection
Vaccines and Immunization
Vaccine
 preparation
of one or more microbial antigens
used to induce protective immunity
 may consist of killed, living, weakened
(attenuated) microbes or inactivated bacterial
toxins (toxoids), purified cell material,
recombinant vectors or DNA
More on Vaccines and
Immunization
Immunization

result obtained when vaccine stimulates immunity
vaccines attempt to induce antibodies and
activated T cells to protect host from future
infection
 Vaccinomics is the application of genomics and
bioinformatics to vaccine development

Table 33.3
Recombinant-Vector Vaccines
pathogen genes that encode major
antigens inserted into nonvirulent viruses
or bacteria which serve as vectors and
express the inserted gene
 released gene products (antigens) can
elicit cellular and humoral immunity
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DNA Vaccines
DNA directly introduced into host cell via air
pressure or gene gun
 when injected into muscle cells, DNA taken into
nucleus and pathogen’s DNA fragment is
expressed
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host immune system responds to foreign proteins
produced
many DNA vaccine trials are currently being run
Table 33.5
Bioterrorism Preparedness
Bioterrorism
 “intentional
or threatened use of viruses,
bacteria, fungi, or toxins from living
organisms to produce death or disease in
humans, animals, and plants”
Examples of Intentional Uses of Biological
Agents for Criminal or Terror Intent

1984 in The Dalles, OR
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1996 in Texas
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intentional release of Shigella dysentariae in a hospital lab
break room
2001 in seven eastern U.S. states
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Salmonella typhimurium in 10 restaurant salad bars
use of weaponized Bacillus anthracis spores delivered through
U.S. postal systems
all of above caused hospitalizations, the anthrax
episode resulted in five deaths
Table 33.6
Choosing Biological Agents as
Weapons
biocrime when chosen as a means for a
localized attack vs. bioterrorism when chosen
for mass casualties
 characteristics that favor their use
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invisible, odorless, and tasteless
difficult to detect
take hours or days before awareness that they have
been used
fear and panic associated with the anticipation that
they were used
Nosocomial Infections
result from pathogens that develop within
a hospital or other clinical care facility and
are acquired by patients while they are in
the facility
 5-10% of all hospital patients acquire a
nosocomial infection
 usually caused by bacteria that are
members of normal microbiota

Source
Endogenous pathogen
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brought into hospital by patient or acquired when
patient is colonized after admission
Exogenous pathogen
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microbiota other than the patient’s
Autogenous infection
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caused by an agent derived from microbiota of
patient despite whether it became part of patient’s
microbiota following admission
Control, Prevention, and
Surveillance

proper training of personnel in basic
infection control measures
 e.g.,
handling of surgical wounds and hand
washing

monitoring of patient for signs and
symptoms of nosocomial infection
The Hospital Epidemiologist
individual responsible for developing and
implementing policies to monitor and
control infections and communicable
diseases
 reports to infection control committee or
similar group
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Bibliography
Lecture PowerPoints Prescott’s Principles
of Microbiology-Mc Graw Hill Co.
 http://en.wikipedia.org/wiki/Scientific_
method
 https://files.kennesaw.edu/faculty/jhend
rix/bio3340/home.html
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