Chapter 14
Disease
&
Epidemiology
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Pathology & Concerns
Pathology - scientific study of disease
concerns for pathology include:
Etiology - the cause of disease
Pathogenesis - the manner in which the disease develops, changes
caused by the disease and the final effects on the body
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Infection & Disease
Infection - invasion or colonization of the body by pathogenic
microorganisms
this differs from…
Disease - when an infection causes a change from the normal state of
health
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Normal Microorganisms
Normal microbiota (flora): microorganisms which colonize the body,
but do not produce disease under normal circumstances (those which
are present, but only temporarily, are transient microbiota)
Normal microbiota can prevent disease or overgrowth of harmful
organisms through competition - called microbial antagonism
This can be direct competition for nutrients, or through production of
bacteriocins, proteins which inhibit other bacteria of similar species
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Microorganisms and Disease
Symbiosis - the host and microbiota living together
This can occur in several formats…
Commensalism - one benefits and the other isn't harmed
Mutualism - both benefit
Parasitism - host is harmed
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Opportunism
Opportunistic pathogens are organisms which do not normally cause
disease in a healthy person, but will cause disease if the person is
weakened
for example, from previous disease, or immunosuppression such as
cancer therapy
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Etiology
Determined via Koch's postulates
1. The same pathogen must be present in every case of the disease.
2. The pathogen must be isolated from the diseased host and grown in
pure culture.
3. The pathogen from the pure culture must cause the disease when it is
inoculated into a healthy, susceptible laboratory animal.
4. The pathogen must be isolated from the inoculated animal and must
be shown to be the original organism.
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Unculturable Organisms
Modifications to Koch's Postulates necessary for unculturable diseases.
Can use molecular methods or compare cultures from different
specimens (mouse, guinea pig etc…) for intracellular parasites
eg.
Treponema pallidum (syphilis)
Mycobacterium leprae (leprosy)
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Classifying Infectious Diseases
Symptoms - subjective changes in body function (pain, malaise - not
easily measurable)
Signs - objective changes that can be observed and measured (eg.
lesions, fever, etc.)
Syndrome - a group of symptoms or signs which may always
accompany a particular disease.
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Classifying Infectious Diseases
Communicable disease - disease which spreads from one host to
another (ex. genital herpes)
Contagious disease - disease which spreads easily from person to
person (ex. chickenpox)
Noncommunicable disease - not spread from host to host
(eg. opportunistic infections caused by normal flora, or disease such as
tetanus introduced from outside the body)
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Occurrence of Disease
Incidence - the fraction of a population that contracts the disease
during a particular time period
Prevalence - fraction of a population having the disease at a specified
time.
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Frequency of Disease
Sporadic - occurs only occasionally
Endemic - constantly present
Epidemic - many people in a given area acquire a certain disease in a
relatively short time period.
Pandemic - worldwide epidemic
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Severity & Duration of Disease
Acute - develops rapidly, but last only a short time (flu)
Chronic - develops more slowly, and often is less severe, but can be
continual or recurrent
Latent - inactive for a time (may be long or short)
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Emerging Infectious Diseases
new or changing diseases showing an increased incidence in the recent
past or a potential to increase in the near future.
Zoonoses – disease of animals which can be transmitted to humans
(Table 14.2, p. 416)
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Extent of Host Involvement
How much of the body is affected?
Local infection - limited to a relatively small portion of the body.
Systemic (generalized) infection - spread throughout the body
bacteremia - bacteria present in blood
septicemia - microorganisms multiply in the blood
toxemia - presence of toxins in the blood (tetanus)
viremia - is viruses in the blood
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Classification of Infection
Primary infection - an acute infection that causes the initial illness.
Secondary infection - an opportunistic pathogen taking advantage of
the weakened defenses due to the primary infection.
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Spread of Infection - Reservoirs
Reservoirs: source of infection, can be living or non-living.
Humans - transmit from person to person. Called carriers, some of
which may not exhibit symptoms.
Animals - (zoonoses) Can occur through contact with the animal, with
animal waste, ingestion of animal products, or by insect vectors.
Non-living - soil and water. (ex. fungi in soil, botulism and tetanus;
water - cholera)
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Transmission of Disease
Contact transmission - direct or indirect contact, or by droplet
transmission. No intermediate. Ex. kissing, touching.
Indirect contact - spread to susceptible host through a non-living
object, called a fomite.
Droplet transmission - mucus droplets spread a short distance (less
than one meter) by talking, sneezing, coughing, etc.
Vehicle transmission - transmission of disease agents by a medium
such as food, water or air (more than 1 meter)
Vector transmission - animals which carry pathogens from one host to
another. (arthropods commonly vectors)
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Nosocomial Infections
Nosocomial Infections - those infections not evident (present or
incubating) at the time of admission to a hospital. (5-15% of all
hospital patients acquire nosocomial infections)
Factors
microorganisms in the hospital
weakened state of the patient (primary infection, drug or radiation
therapy, burns or surgery)
chain of transmission in the hospital - can transfer patient to patient,
staff to patient
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used to be mainly Gram-positive, but now mostly Gram-negatives
Nosocomial
infection
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Control of Nosocomials
preventative procedures
Good aseptic technique
isolation
use of disposable or carefully sterilize equipment before reusing
monitoring procedures to trace causes.
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Development of a Disease
Incubation period - time between infection and appearance of
symptoms or signs.
Prodromal period - in some diseases, short period of mild symptoms.
Period of Illness - most acute. Exhibits overt signs and symptoms. If
not successfully overcome, the patient dies during this period. If
overcome, then enter the
Period of Decline - signs and symptoms diminish, susceptible to
secondary infections.
Period of Convalescence - regains strength and returns to prediseased
state
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Incubation
Period
Death if immune response
or medical intervention fail
Illness
Prodromial
Period
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Disease Decline
Convalescence
Epidemiology
Epidemiology - study of when and where diseases occur and how they
are transmitted in populations.
Descriptive epidemiology - collection of data that describe the
occurrence of the disease under study.
Analytical epidemiology - analyzes a particular disease to determine
its probable cause. (risk factors, etc.)
Experimental epidemiology - makes hypothesis about a particular
disease and experiments to test this hypothesis with a group of people.
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CDC
Case reporting of certain disease cases to the state and national level.
Used to keep track of the spread of diseases.
Centers for Disease Control and Prevention (CDC) - central source of
epidemiological information in the U.S. Publishes Morbidity and
Mortality Weekly Report.
Morbidity - incidence of specific diseases
Mortality - number of deaths from these diseases.
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