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Chapter 14
Infection,
Infectious
Diseases, and
Epidemiology
© 2012 Pearson Education Inc.
Lecture prepared by Mindy Miller-Kittrell
North Carolina State University
Symbiotic Relationships Between Microbes and Their Hosts
• Symbiosis means “to live together”
• We have symbiotic relationships with
countless microorganisms
• Types of symbiosis
– Mutualism
– Commensalism
– Parasitism, pathogen
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Table 14.1 The Three Types of Symbiotic Relationships
Figure 14.1 Which type of symbiotic relationship?
Symbiotic Relationships Between Microbes and Their Hosts
• Normal Microbiota in Hosts
– Also termed normal flora and indigenous
microbiota
– Organisms that colonize the body’s surfaces
without normally causing disease
– Two types
– Resident microbiota
– Transient microbiota
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Figure 14.2 Normal microbiota of the human nasal cavity
Symbiotic Relationships Between Microbes and Their Hosts
• Normal Microbiota in Hosts
– Resident microbiota
– Are a part of the normal microbiota throughout life
– Are mostly commensal
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Table 14.2 Some Resident Microbiota
Symbiotic Relationships Between Microbes and Their Hosts
• Normal Microbiota in Hosts
– Transient microbiota
– Remain in the body for short period
– Found in the same regions as resident microbiota
– Cannot persist in the body
– Competition from other microorganisms
– Elimination by the body’s defense cells
– Chemical or physical changes in the body
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Symbiotic Relationships Between Microbes and Their Hosts
• Normal Microbiota in Hosts
– Acquisition of normal microbiota
– Development in womb free of microorganisms
(axenic)
– Microbiota begin to develop during birthing
process
– Much of one’s resident microbiota established
during first months of life
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Symbiotic Relationships Between Microbes and Their Hosts
• How Normal Microbiota Become Opportunistic
Pathogens
– Opportunistic pathogens
– Normal microbiota that cause disease under certain
circumstances
– Conditions that provide opportunities for pathogens
– Introduction of normal microbiota into unusual site
in body (ex. E. coli)
– Immune suppression
– Changes in the normal microbiota
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Reservoirs of Infectious Diseases of Humans
• Most pathogens cannot survive for long
outside their host
• Reservoirs of infection
– Sites where pathogens are maintained as a
source of infection
• Three types of reservoirs
– Animal reservoirs
– Human carriers
– Nonliving reservoirs
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Reservoirs of Infectious Diseases of Humans
• Animal Reservoirs
– Zoonoses
– Diseases naturally spread from animal host to
humans
– Acquire zoonoses through various routes
– Direct contact with animal or its waste
– Eating animals
– Bloodsucking arthropods
– Humans are usually dead-end host to zoonotic
pathogens
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Reservoirs of Infectious Diseases of Humans
• Human Carriers
– Infected individuals who are asymptomatic
but infective to others
– Some individuals eventually develop illness
while others never get sick
– Healthy carriers may have defensive systems
that protect them
– P. 416 A deadly carrier
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Reservoirs of Infectious Diseases of Humans
• Nonliving Reservoirs
– Soil, water, and food can be reservoirs of
infection
– Presence of microorganisms often due to
contamination by feces or urine
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The Movement of Microbes into Hosts: Infection
• Exposure to Microbes: Contamination and
Infection
– Contamination
– The mere presence of microbes in or on the body
– Infection
– When organism evades body’s external defenses,
multiplies, and becomes established in the body
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The Movement of Microbes into Hosts: Infection
• Portals of Entry
– Sites through which pathogens enter the
body
– Four major pathways
– Skin
– Mucous membranes
– Placenta
– Parenteral route
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Figure 14.3 Routes of entry for invading pathogens
Ear
Broken
skin
Insect
bite
Conjunctiva
of eye
Nose
Mouth
Placenta
Vagina
Anus
In males:
Penis
Urethra
The Movement of Microbes into Hosts: Infection
• Portals of Entry
– Skin
– Outer layer of dead skin cells acts as a barrier to
pathogens
– Some pathogens can enter through openings
or cuts
– Others burrow into or digest outer layers of skin
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Figure 14.4 A cross-section of skin
Sebaceous
(oil) gland
Hair
Pore of
sweat duct
Epidermis
Blood
capillaries
Nerve
fiber
Dermis
Nerve
ending
Vein
Artery
Sweat
gland
Fat cells
Hair
follicle
The Movement of Microbes into Hosts: Infection
• Portals of Entry
– Mucous membranes
– Line the body cavities that are open to the
environment
– Provide a moist, warm environment hospitable to
pathogens
– Respiratory tract is the most common site of entry
– Entry is through the nose, mouth, or eyes
– Gastrointestinal tract may be route of entry
– Must survive the acidic pH of the stomach
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The Movement of Microbes into Hosts: Infection
• Portals of Entry
– Placenta
– Typically forms effective barrier to pathogens
– Pathogens may cross the placenta and infect
the fetus
– Can cause spontaneous abortion, birth
defects, premature birth
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The Movement of Microbes into Hosts: Infection
• Portals of Entry
– Parenteral route
– Not a true portal of entry
– Means by which the portal of entry can be
circumvented
– Pathogens deposited directly into tissues
beneath the skin or mucous membranes
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The Movement of Microbes into Hosts: Infection
• The Role of Adhesion in Infection
– Process by which microorganisms attach
themselves to cells
– Required to successfully establish colonies
within the host
– Uses adhesion factors
– Specialized structures
– Attachment proteins
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Figure 14.5 The adhesion of pathogens to host cells-overview
The Movement of Microbes into Hosts: Infection
• The Role of Adhesion in Infection
– Attachment proteins help in adhesion
– Found on viruses and many bacteria
– Viral or bacterial ligands bind host cell receptors
– Interaction can determine host cell specificity
– Changing/blocking a ligand or its receptor can
prevent infection
– Inability to make attachment proteins or adhesins
renders microorganisms avirulent
– Some bacterial pathogens attach to each other to
form a biofilm
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Figure 14.6 Dental plaque
The Nature of Infectious Disease
• Infection is the invasion of the host by a
pathogen
• Disease results if the invading pathogen
alters normal body functions
• Disease is also referred to as morbidity
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The Nature of Infectious Disease
• Manifestations of Disease: Symptoms, Signs,
and Syndromes
– Symptoms
– Subjective characteristics of disease felt only by the
patient
– Signs
– Objective manifestations of disease observed or
measured by others
– Syndrome
– Symptoms and signs that characterize a disease or
abnormal condition
– Asymptomatic, or subclinical, infections lack
symptoms but may still have signs of infection
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The Nature of Infectious Disease
• Causation of Disease: Etiology
– Study of the cause of disease
– Germ theory of disease
– Disease caused by infections of pathogenic
microorganisms
– Robert Koch developed a set of postulates to
prove a particular pathogen causes a
particular disease
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Figure 14.7 Koch’s postulates
Agent not typically found
in healthy subjects
The suspected agent must be present
in every case of the disease.
Diseased subjects
Healthy subjects
Petri plate
Bacterial
colonies
The agent must be
isolated and grown
in pure culture.
Streaked plates
Injection
The cultured agent must cause
the disease when it is inoculated
into a healthy, susceptible
experimental host (animal or plant).
The same agent must
be reisolated from the
diseased experimental
host.
The Nature of Infectious Disease
• Causation of Disease: Etiology
– Exceptions to Koch’s postulates
– Some pathogens can’t be cultured in the
laboratory
– Diseases caused by a combination of pathogens
and other cofactors
– Pathogens that require a human host
– Difficulties in satisfying Koch’s postulates
– Diseases can be caused by more than one
pathogen
– Pathogens that are ignored as potential causes of
disease
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The Nature of Infectious Disease
• Virulence Factors of Infectious Agents
– Pathogenicity
– Ability of a microorganism to cause disease
– Virulence
– Degree of pathogenicity
– Virulence factors contribute to virulence
– Adhesion factors
– Biofilms
– Extracellular enzymes
– Toxins
– Antiphagocytic factors
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Figure 14.8 Relative virulence of some microbial pathogens
More virulent
Francisella tularensis
(rabbit fever)
Yersinia pestis
(plague)
Bordetella pertussis
(whooping cough)
Pseudomonas aeruginosa
(infections of burns)
Clostridium difficile
(antibiotic-induced colitis)
Candida albicans
(vaginitis, thrush)
Lactobacilli, diphtheroids
Less virulent
The Nature of Infectious Disease
• Virulence Factors of Infectious Agents
– Extracellular enzymes
– Secreted by the pathogen
– Dissolve structural chemicals in the body
– Help pathogen maintain infection, invade, and
avoid body defenses
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Figure 14.9a Some virulence factors: Extracellular enzymes
Hyaluronidase and collagenase
Bacterium
Coagulase and kinase
Bacterium
Hyaluronidase
Coagulase
Clot
Clotting
protein
Epithelial
cells
Kinase
Collagenase
Collagen layer
Invasive bacteria
reach epithelial
surface.
Bacteria produce
hyaluronidase and
collagenase.
Extracellular enzymes
Bacteria invade deeper
tissues.
Bacteria produce
coagulase.
Clot forms.
Bacteria later produce
kinase, dissolving clot
and releasing bacteria.
The Nature of Infectious Disease
• Virulence Factors of Infectious Agents
– Toxins
– Chemicals that harm tissues or trigger host
immune responses that cause damage
– Toxemia refers to toxins in the bloodstream that
are carried beyond the site of infection
– Two types
– Exotoxins
– Endotoxins
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Figure 14.9b Some virulence factors: Toxins
Exotoxin
Bacterium
Endotoxin
Exotoxin
Phagocyte
Phagocytized
Gram bacteria
Exocytosis
Endotoxin
Dead Gram
bacteria
Blood vessel
Bacteria secrete exotoxins, in this Cytotoxin kills host’s cells.
case a cytotoxin.
Toxins
Dead Gram-negative bacteria release endotoxin (lipid A), which induces
effects such as fever, inflammation, diaarrhea, shock, and blood coagulation.
The Nature of Infectious Disease
ANIMATION Virulence Factors: Exotoxins
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The Nature of Infectious Disease
ANIMATION Virulence Factors: Endotoxins
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The Nature of Infectious Disease
• Virulence Factors of Infectious Agents
– Antiphagocytic factors
– Factors prevent phagocytosis by the host’s
phagocytic cells
– Bacterial capsule
– Composed of chemicals not recognized as foreign
– Slippery
– Antiphagocytic chemicals
– Prevent fusion of lysosome and phagocytic vesicles
– Leukocidins directly destroy phagocytic white
blood cells
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Figure 14.9c Some virulence factors: Antiphagocytic factors
Phagocytosis blocked by capsule
Capsule around
bacterium
Incomplete phagocytosis
Capsule around
bacterium
Phagocytic
vesicle
Phagocyte
Lysosome
Antiphagocytic factors
Bacteria
reproduce
The Nature of Infectious Disease
ANIMATION Virulence Factors: Hiding from
Host Defenses
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The Nature of Infectious Disease
• The Stages of Infectious Disease
– The disease process occurs following infection
– Many infectious diseases have five stages
following infection
– Incubation period
– Prodromal period
– Illness
– Decline
– Convalescence
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Number of microorganisms or
intensity of signs or symptoms
Figure 14.10 The stages of infectious diseases
Incubation Prodromal
period
period
(vague,
(no signs or
general
symptoms)
symptoms)
Illness
(most severe signs
and symptoms)
Time
Decline
(declining signs
and symptoms)
Convalescence
(no signs or
symptoms)
The Movement of Pathogens Out of Hosts: Portals of Exit
• Pathogens leave host through portals of exit
• Many portals of exit are the same as portals
of entry
• Pathogens often leave hosts in materials the
body secretes or excretes
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Figure 14.11 Portals of exit
Eyes
(tears)
Ear
(earwax)
Nose
(secretions)
Broken skin
(blood)
Mouth
(saliva, sputum)
Skin
(flakes)
In females:
Mammary glands
(milk, secretions)
Vagina
(secretions, blood)
Anus
(feces)
Seminal vesicles
(semen and lubricating secretions)
Urethra
(urine)
Modes of Infectious Disease Transmission
• Transmission is from a reservoir or a portal
of exit to another host’s portal of entry
• Three groups of transmission
– Contact transmission
– Direct, indirect, or droplet
– Vehicle transmission
– Airborne, waterborne, or foodborne
– Vector transmission
– Biological or mechanical
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Modes of Infectious Disease Transmission
ANIMATION Epidemiology: Transmission of Disease
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Figure 14.12 Droplet transmission
Figure 14.13 Poorly refrigerated foods can harbor pathogens and transmit diseases
Classification of Infectious Diseases
• Diseases can be classified in number of ways
–
–
–
–
The body system they affect
Their longevity and severity
How they are spread to their host
The effects they have on populations (rather than
on individuals)
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Classification of Infectious Diseases
• Terms used to classify infectious disease
–
–
–
–
–
–
Acute disease
Chronic disease
Subacute disease
Latent disease
Communicable
Contagious
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Epidemiology of Infectious Diseases
ANIMATION Epidemiology: Overview
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Epidemiology of Infectious Diseases
• Frequency of Disease
– Track occurrence of diseases using two
measures
– Incidence
– Number of new cases of a disease in a given area
during a given period of time
– Prevalence
– Number of total cases of a disease in a given area
during a given period of time
– Occurrence also evaluated in terms of frequency
and geographic distribution
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Incidence (thousands)
Incidence
Year
Prevalence
Prevalence (thousands)
Figure 14.14 Curves representing the incidence and the estimated prevalence of AIDS among U.S. adults
Figure 14.15 Incidence of shigellosis in the U.S. in 2000-overview
Figure 14.16 Illustrations of the different terms for the occurrence of disease-overview
Epidemiology of Infectious Diseases
ANIMATION Epidemiology: Occurrence of Disease
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Number of cases per 100,000 individuals
Figure 14.17 Epidemics may have fewer cases than nonepidemics
Epidemic 2
Epidemic 1
Time
Expected number of cases
Actual number of cases Disease A
Actual number of cases Disease B
Figure 14.18 A page from the CDC’s Morbidity and Mortality Weekly Report (MMWR)
Epidemiology of Infectious Diseases
• Epidemiological Studies
– Descriptive epidemiology
– Careful tabulation of data concerning a disease
– Record location and time of the cases of disease
– Collect patient information
– Try to identify the index case (or first case) of
the disease
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Figure 14.19 A map showing cholera deaths in a section of London, 1854
Deaths from cholera
Pump
Epidemiology of Infectious Diseases
• Epidemiological Studies
– Analytical epidemiology
– Seeks to determine the probable cause, mode of
transmission, and methods of prevention
– Useful in situations when Koch’s postulates can’t
be applied
– Often retrospective
– Investigation occurs after an outbreak has occurred
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Epidemiology of Infectious Diseases
• Epidemiological Studies
– Experimental epidemiology
– Involves testing a hypothesis concerning the
cause of a disease
– Application of Koch’s postulates is
experimental epidemiology
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Epidemiology of Infectious Diseases
ANIMATION Nosocomial Infections: Overview
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Epidemiology of Infectious Diseases
• Hospital Epidemiology: Nosocomial
Infections
– Types of nosocomial infections
– Exogenous
– Pathogen acquired from the health care environment
– Endogenous
– Pathogen arises from normal microbiota due to
factors within the health care setting
– Iatrogenic
– Results from modern medical procedures
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Figure 14.20 The interplay of factors that result in nosocomial infections
Immunocompromised
patients
Presence of
microorganisms in
hospital environment
Nosocomial
infection
Transmission of
pathogens between
staff and patients
and among patients
Epidemiology of Infectious Diseases
• Hospital Epidemiology: Nosocomial
Infections
– Control of nosocomial infections
– Precautions designed to reduce factors that result
in disease
– Hand washing is the most effective way to reduce
nosocomial infections
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Epidemiology of Infectious Diseases
ANIMATION Nosocomial Infections: Prevention
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Epidemiology of Infectious Diseases
• Epidemiology and Public Health
– Local, state, national, and global agencies share
information concerning disease
– The United States Public Health Service
– World Health Organization (WHO)
– Public health agencies work to limit disease
transmission
– Monitor water and food safety
– Public health agencies campaign to educate the
public on healthful choices to limit disease
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