Epidemiology of Infectious disease
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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
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Symbiotic Relationships Between Microbes and Their Hosts
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Infection
 Mutualsim: Each one benefits
 Commensual: One organism benefits, the other is
not harmed.
 Parasitism: Advantage to one, disadvantage to the
other.
 Obligate relationship: one depends on the other for
survival (parasites are obligate pathogens)
 Normal Flora: Organism is always on or in us; GI, GU, oral
mucosa.
 Sterile areas of the body: Blood, CSF. Any organism here
indicates something is wrong. The first indicator is
decreased glucose, increased protein
 Opportunistic Pathogens: A normal flora organism that
becomes a pathogen when the host’s immune defense
decreases.

Trauma, Surgery, Burns: overcomes barriers of defense

Catheterization: allows increased access and decreased defense
Chemotherapy: e.g. ablative (for CA) destroys bone marrow,
therefore myeloid and lymphoid immunity goes down.
Co-infection: e.g. haemophilic influenza is 2° to viral influenza.

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Tissue Tropism
 Tissue Tropism (organisms are selective of tissues they
infect): influenza does not affect your hand; gonorrhea
only effects mucosa. Tropism can also be species, organ,
or non-specific.
 HIV integrates a provirus into our chromosome.
Antibodies actually assist the HIV organisms because
opsinization helps to internalize the virus. Because of
almost no cell mediated immunity (T-cells are infected),
AIDS patients die from TB, cytomegalovirus (CMV),
Candida, pneumocystis. When a person has a second
infection, it is called a CO-INFECTION.
OBLIGATE PATHOGENS
 Will always cause infection and disease whenever
and wherever they are encountered (e.g. Yersinia
pestis plague, anthrax, smallpox) unless you are
immune. However, they don’t want to kill their
host.
Virulence
 VIRULENCE: Ability to cause infection and disease. They have special
structures that allow them to attach, invade, spread, etc.
 MULTIPLICITY OF INFECTION (INNOCULUM): the number of
organisms needed to cause disease. There is an inverse relationship
between virulence and Multiplicity of Infection. If only a few bacterial cells
cause the disease, it is said to be very virulent.
 Obligate pathogens are virulent, even with a low inoculum, because they
are effective. Most innoculum needs 103-5 organisms. Only 10 organisms
can form an innoculum in a virulent organism such as anthrax.
 Anthrax avoids immune defenses by exploiting the very mechanisms
used to fight it. It becomes activated after phagocytosis.
 Mycobacterium will coat the wall of vacuoles so lysozymes can’t fuse.
 Rickettsia is on a timer, and escapes the vacuole to invade the nucleus.
PORTALS OF ENTRY (POE)
 CEPHALIC (7 PORTALS): Mouth, nose, eyes, ears.
 CORPOREAL: Mammary, Vaginal, Urethral, Rectal
 TRAUMA/MEDICAL: Burn, Compound Fracture,
surgical/catheter, injury/IVDA, abnormal mucosa (CA
chemo).
MODES OF TRANSMISSION (MOT)
 AEROSOL: Airborne micro-particles (soil aerosol containing
endospores)
 DROPLET NUCLEI: Mucoid micro-droplet via cough or sneeze, durable
on surfaces, mainly transmitted by contact. Sneeze and cough into your
elbow!
 DIRECT CONTACT: Direct object/tissue-to-tissue contact.
o
CASUAL TRANSMISSION: handshake, clothing
o
ASPIRATION: inhalation of oral, GI, or food-borne organisms
o
FECAL-ORAL: autoinoculation or contamination
(poor hygiene)
o
SEXUAL TRANSMISSION (STD): sexual/bodily fluid contact
AGENTS OF TRANSMISSION
 VECTOR: a living organism that spreads disease from one
host to another.
(mosquitoes, ticks, rats, humans)
 FOMITE: inanimate object that spreads disease from one
host to another.
(toothbrush, water glass, toys, handles, keyboard, light
switch, cell phones, TV remote)
HOST-PATHOGEN RELATIONSHIP (HPR)
 INFECTION: presence of organism
 DISEASE: adverse symptoms due to infection
 NORMAL FLORA: organism typically found on healthy individual
 PATHOGEN: organism causing disease (etiologic agent)
 IMMUNOCOMPROMISED: suppressed or deficient immunity
 NOSOCOMIAL: acquired in hospital setting
 COMMUNITY-ACQUIRED: acquired in routine, day-to-day activities
 OPPORTUNISTIC PATHOGEN: normal flora + compromised  pathogen
 OBLIGATE PATHOGEN: always causes Dz if present
 PATHOGENESIS (Px): events leading to Dz (POEDz)
 MULTIPLICITY OF INFECTION (MOI): inoculum required for infxn
 INCUBATION (INC): time from transmission to Dz
 VIRULENCE: measure of Dz capability
 VIRULENCE FACTOR (VF): ultrastructure, exoenzyme, or exotoxin
BACTERIAL SYMPTOM TERMINOLOGY
 BACTEREMIA: bacteria in bloodstream
 BACTERURIA: bacteria in urine (normal or pathogenic flora)
 SEPTICEMIA: Gram negative bacteremia in the bloodstream
 PYEMIA: Gram + bacteremia
 PYROGENIC: fever-producing infection or bacterial components
 PURULENT: pus-producing, reaches an opening to drain out. The pus is a
collection of dead neutrophils, which are the 1° immune cells when there is pus.
 GRANULOMATOUS: infxn where macrophages are 1° immune cells (no pus)
 DIFFERENTIAL DIAGNOSIS (DDx): all possible causes of the disease.
 UROSEPSIS: Septicemia from UTI
 FACULTATIVE INTRACELLULAR PARASITES: are not obligate pathogens. They
can survive temporarily in white cells, such as neutrophils, Monocytes/
Macrophages.
 EXOTOXINS: A toxin made and then secreted, e.g. pesticin, hemolysins,
shigatoxins, choleragen, enterotoxins, plasminogen activating factor, coagulase,
etc.
 ENDOTOXINS: A component of the cell wall is toxic to us, e.g. LPS and LOS.
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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Symbiotic Relationships Between Microbes and Their Hosts
 Resident microbiota
 Are a part of the normal microbiota throughout life
 Are mostly commensal
 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
 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
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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
 Immune suppression
 Changes in the normal microbiota
 Changes in relative abundance may allow
opportunity for a member to thrive and cause
disease
Inflammation
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Host defense summary
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What can you catch
from a toilet seat?
Cell surface markers
 Designated as “CD”
 Cluster of Differentiation
 used to identify specific structures on a cell
 ~200 different designations
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Reservoirs of Infectious Diseases of Humans
 Most pathogens cannot survive for long outside of
their host
 Reservoirs of infection
 Sites where pathogens are maintained as a source
of infection
 Three types of reservoirs
 Animal reservoir
 Human carriers
 Nonliving reservoir
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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
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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 (intravenous)
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Routes by which humans acquire parasitic infections
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Figure 23.1
Routes of entry for invading pathogens
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Portals of exit
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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 enter by burrowing into or digesting
outer layers of skin
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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
 Pathogens deposited directly into tissues beneath
the skin or mucous membranes (needle injection
or i.v.)
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The Movement of Microbes into Hosts:
Infection
 The Role of Adhesion in Infection
 Process by which microorganisms attach
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themselves to cells
 Required to successfully establish colonies within
the host
 Adhesion factors include
 Specialized structures (acts like Velcro)
 Attachment proteins (acts like glue)
 The presence of adhesions are considered a
virulence factor.
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 (proteins) bind host cell
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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
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
 Cause 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 one
must satisfy to prove a particular pathogen causes a
particular disease
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Koch’s Postulates
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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
 Ethical considerations prevent applying Koch’s
postulates to 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 (how good is it at its job?)
 Virulence factors contribute to virulence
 Adhesion factors
 Biofilms
 Extracellular enzymes
 Toxins
 Antiphagocytic factors
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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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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 (made and secreted)
 Endotoxins (part of the bacterial cell wall)
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The Nature of Infectious Disease
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The Nature of Infectious Disease
 Virulence Factors of Infectious Agents
 Antiphagocytic factors
 Factors prevent phagocytosis by the host’s phagocytic
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cells
 Bacterial capsule
o Composed of chemicals not recognized as foreign
o Slippery; difficult for phagocytes to engulf bacteria
 Antiphagocytic chemicals
o Prevent fusion of lysosome and phagocytic vesicles
o Leukocidins directly destroy phagocytic white blood
cells
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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The stages of infectious disease
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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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Portals of exit
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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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Classification of Infectious Diseases
 Diseases can be classified in number of ways
 The body system they affect
 GI system, nervous system, blood
 Taxonomic categories
 Viral disease, Gram +
 Their longevity and severity
 Chronic vs. acute; mild vs. severe
 How they are spread to their host
 STD, blood borne, food
 The effects they have on populations (rather than
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on individuals): global/pandemic vs. epidemic
Classification of Infectious Diseases
 Terms used to classify infectious disease
 Acute disease (rapid onset, short duration)
 Chronic disease (long duration)
 Subacute disease (slower onset, medium duration)
 Latent disease (present but not obvious symptoms)
 Communicable (Contagious)
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Epidemiology of Infectious Diseases
INFECTION: Presence of microbe
DISEASE: Symptomatic
consequence of infection
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Epidemiology
 Epidemiology is the study of the patterns, causes, and effects
of health and disease conditions in defined populations. It is
the cornerstone of public health by identifying risk factors
for disease and provides information for preventive
healthcare.
 Major areas of epidemiological study include disease etiology,
outbreak investigation, disease surveillance and screening,
biomonitoring, and comparisons of treatment effects such as
in clinical trials.
John Snow
 Dr. John Snow is
known as the father
of modern
epidemiology. He
showed that cholera
was water-borne,
and debunked the
Miasma theory,
which stated that
cholera came from
bad air.
John Snow
 John Snow was a British physician in London in 1854, where
a major cholera outbreak struck the nearby city of Soho.
 In three days, 127 people who lived near Broad Street died of
Cholera. Within a week, 500 people had died and one in
seven people who developed cholera eventually died from it.
 John Snow suspected contamination of the water, but
couldn’t find any organic matter in it. Almost all the cases of
cholera occurred close to the Broad Street Water Pump.
 Snow mapped out the cases of cholera in relation to the
water pump.
 The evidence Snow presented convinced the local council to
remove the pump handle, halting the spread of the disease. It
wasn’t until 1885, when Robert Koch identified V. cholerae as the
bacillus causing the disease that he had proof of his theory. He was
right, but wasn’t around to see this discovery himself. John Snow
died four years later, at the age of 45.
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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The incidence and estimated
prevalence of AIDS
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Epidemiologists report data in a
number of ways
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Different terms for the occurrence of
disease
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Epidemics Defined Relative to
Expected Number of Cases
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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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Epidemiology of Infectious Diseases
 Epidemiological Studies
 Analytical epidemiology
 Seeks to determine the probable cause
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(etiology), 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
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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Endemic
1: characteristic of or prevalent in a particular field, area, or
environment
2: restricted or peculiar to a locality or region
 “Endemic" means "in the population."
 "Endemic" is often used to characterize diseases that are
generally found in a particular area; malaria, for example, is
said to be endemic to tropical and subtropical regions. This use
differs from that of the related word "epidemic" in that it
indicates a more or less constant presence in a particular
population or area rather than a sudden, severe outbreak within
that region or group. The word is also used by biologists to
characterize the plant and animal species that are only found in
a given area.
Epidemic
 Affecting or tending to affect a DISPROPORTIONATELY large
number of individuals within a population, community, or
region at the same time
Demographics
 The changes (such as the number of births, deaths,
marriages, and illnesses) that occur over a period of time in
human populations; also : a set of such changes.
 The demographics of a particular disease would include the
ratio of males to females that have the disease, the average
age, the geographical distribution, and the incidence (the
likelihood of a person getting the disease) in a particular
Country.
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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The interplay of factors that result in
nosocomial infections
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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
 Epidemiology and Public Health
 Agencies at the local, state, national, and global
level 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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Basic Principles of
Microbial Control