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CHAPTER
14
Principles of
Disease and
Epidemiology
© 2016 Pearson Education, Ltd.
Pathology, Infection, and Disease
Definitions
 Pathology (病理學): The study of disease
pathos = suffering
logos = science
 Etiology (病原學): The study of the cause of a disease
 Pathogenesis (發病): The development of disease
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Pathology is concerned with
(1) the etiology (cause),
(2) pathogenesis (development),
(3) effects (structural and functional changes) of disease.
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More definition
 Infection: Colonization of the body by pathogens
 Disease: An abnormal state in which the body is not
functioning normally
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Normal Microbiota
(微生物相/群)
 Animals, including humans, are generally free of
microbes in utero (子宮).
 Microorganisms begin colonization in and on the surface
of the body soon after birth.
 Just before a woman gives birth, lactobacilli in mother’s
vagina (陰道) multiply rapidly. It becomes the
newborne’s first contact and become predominant in
baby’s intestine. After birth, E. coli then inhabit the
large intestine.
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 Many other usually harmless microorganisms establish
themselves inside other parts of the normal adult body
and on its surface. A typical human body contains 1 x
1013 body cells, yet harbors an estimated 1 x 1014
bacterial cells.
 The Human Microbiome Project began in 2007 to
analyze microbial communities call micromes that live
in and out of human body.
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 Microorganisms that establish more or less permanent
colonies inside or on the body without producing
disease make up the normal microbiota (or normal flora,
正常微生物相).
Bacteria (orange
spheres) on the surface of
the nasal epithelium
Bacteria (brown)
on the lining of the stomach
Bacteria (orange)
in the small intestine
Figure 14.1 Representative normal microbiota for different regions of the body.
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 Transient microbiota (短暫、過渡微生物相) may be
present for days, weeks, or months and then disappear.
 Microorganisms are not found throughout the entire
human body but are localized in certain regions.
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 Locations of normal
microbiota on and in
the human body.
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舊版Table 14.1附圖
Table 14.1 Representative Normal Microbiota by Body Region (Part 1 of 2)
結膜
Figure 14.1
Normal microbiota for different regions of the
body
Table 14.1 Representative Normal Microbiota by Body Region (Part 2 of 2)
Relationship between the Normal Microbiota
and the Host
 Microbial antagonism (頡抗): the normal
microbiota can prevent pathogens from causing an
infection.
 Symbiosis (共生): the relationship between normal
microbiota and the host.
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 Microbial antagonism (competitive exclusion):
 Involves competition (競爭) among microbes.
 Compete for
 Nutrient
 Produce substance harmful to the invading microbes
 Affect conditions such as pH, oxygen
 For example:
 Normal microbiota vs Candida albicans in vagina
 E. coli vs Salmonella and Shigella in large intestine
 Normal microbiota vs Clostridium difficiles in large
intestine
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 Symbiosis:
 Three types of symbiosis are
 Commensalism (片利共生): one organism benefits and the
other is unaffected). Ex: Corynebacteria inhabit the
surface of the eye.
 Mutualism (互利共生): both organisms benefit. Ex: E. coli
in the large intestine synthesize Vit K and Vit B.
 Parasitism (寄生): one organism benefits and one is
harmed. Ex: Many disease-causing bacteria.
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Figure 14.2 Symbiosis.
SYMBIOSIS
One organism
benefits, and the other is unaffected
Staphylococcus
epidermidis on the skin
Both organisms benefit
E. coli bacteria
(lavender) in the large
intestine
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One organism benefits
at the expense of the other
H1N1 virus particles
(orange) on a host
cell (green)
Opportunistic Microorganisms (伺機性微生物)
 Opportunists (opportunistic pathogens) do not cause
disease under normal conditions but cause disease
under special conditions.
 For example: Pneumocystis jirovecii (卡氏肺囊蟲) in
AIDS patients; echoviruses, adenoviruses, Neisseria
meningitidis and Streptococcus pneumoniae in healthy
individuals.
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Cooperation among Microorganisms
 In some situations, one microorganism makes it
possible for another to cause a disease or to produce
more severe symptoms; this is called cooperation.
 For example: oral streptococci in teeth.
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The Etiology of Infectious Diseases
Koch’s Postulates (科霍氏法則)
 Koch’s postulates are a method for establishing that
specific microbes cause specific diseases.
 Koch’s postulates have the following requirements:
1. the same pathogen must be present in every case of the
disease.
2. the pathogen must be isolated in pure culture.
3. the pathogen isolated from pure culture must cause the
same disease in a healthy, susceptible laboratory animal.
4. the pathogen must be reisolated from the inoculated
laboratory animal.
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 Koch's postulates are used to prove the cause of an
infectious disease.
Microorganisms are
isolated from a diseased or
dead animal.
The microorganisms are
grown in pure culture.
Microorganisms are
inoculated into a healthy
laboratory animal
Disease is reproduced in a
laboratory animal.
The microorganisms are
isolated from this animal and
grown in pure culture.
Colony
KEY CONCEPTS
According to Koch's postulates,
a specific infectious disease is
caused by a specific microbe.
The microorganism from the
diseased host caused the same
disease in a laboratory host.
Koch's postulates help determine
the etiology of disease, the first step
in
treatment and prevention.
Microbiologists use these steps
to identify causes of emerging
diseases.
Microorganisms are
identified.
Figure 14.3 Koch's Postulates: Understanding Disease.
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Microorganisms are
identified.
Exceptions to Koch’s Postulates
 Koch’s postulates are modified to establish etiologies of
diseases caused by viruses and some bacteria, which
cannot be grown on artificial media.
 Some microbes have unique culture requirements or have
never been cultured. Ex: Treponema pallidum, pathogen of
syphilis (梅毒)
 Some diseases are not as clear-cut. Ex: nephritis (腎炎)
 Some pathogens cause several disease conditions. Ex:
Mycobacterium tuberculosis, S. pyogenes (化膿鏈球菌)
 Certain pathogens cause disease in humans only. Ex: HIV
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Classifying Infectious Diseases
 Symptom (症狀): A change in body function, such as
pain and malaise, that is felt by a patient as a result of
disease.
 Sign (病徵): A change in a body that can be measured
or observed as a result of disease.
 Syndrome (症候): A specific group of signs and
symptoms that accompany a disease.
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 Communicable disease (傳染病): A disease that is
spread from one host to another, either directly or
indirectly.
 Contagious disease (傳染病): A disease that is easily
spread from one host to another.
 Noncommunicable disease: A disease that is not
transmitted from one host to another.
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Occurrence of a Disease
 Incidence (發生率): the number of people in a
population who develop a disease during a particular
time period. It is an indicator of the spread of a disease.
 Prevalence (流行率): the number of people in a
population who develop a disease during a particular
time period, regardless of when it first appeared.
For example, the incidence of AIDS in the US in 2012
was 55,400 whereas the prevalence in that same year
was estimated to be about 117,000.
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Figure 14.4 Reported AIDS cases in the United States.
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 Diseases are classified by frequency of occurrence:
 Sporadic disease (偶發性疾病): Disease that occurs
occasionally in a population.
 Endemic disease (地方性疾病): Disease constantly
present in a population.
 Epidemic disease (流行性疾病): Disease acquired by
many hosts in a given area in a short time.
 Pandemic disease (世界流行性疾病): Worldwide epidemic.
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Severity or Duration of a Disease
 The scope of a disease:
 Acute disease (急性疾病): Symptoms develop rapidly.
 Chronic disease (慢性疾病): Disease develops slowly.
 Subacute disease (次急性疾病): Symptoms between acute
and chronic.
 Latent disease (潛伏性疾病): Disease with a period of no
symptoms when the patient is inactive.
 Herd immunity (群免疫力): Immunity in most of a population.
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Extent of Host Involvement
 Local infection (局部性感染): Pathogens are limited to a
small area of the body.
 Systemic infection (系統性感染): An infection
throughout the body.
 Focal infection (病灶傳染): Systemic infection that
began as a local infection.
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 Bacteremia (菌血症): Bacteria in the blood.
 Septicemia (敗血症): Growth of bacteria in the blood.
 Toxemia (毒血症): Toxins in the blood.
 Viremia (病毒血症): Viruses in the blood.
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 The state of host resistance also determines the extent
of infection.
 Primary infection: Acute infection that causes the
initial illness.
 Secondary infection: Opportunistic infection after a
primary (predisposing) infection.
 Subclinical (inapparent) disease: No noticeable signs or
symptoms.
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Patterns of Disease
Predisposing Factors (預傾向因素)
 A predisposing factor is one that makes the body more
susceptible to disease or alters the course of a disease.
 Gender. Ex: Short urethra that infect urinary tract
infection in females.
 Inherited traits. Ex: the sickle-cell gene.
 Climate and weather
 Others: inadequate nutrition, fatigue, age, environment,
habits, lifestyle, occupation, preexisting illness,
chemotherapy, and emotional disturbances……
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Development of Disease
1. The incubation period (潛伏期) : the time interval between
the initial infection and the first appearance of signs and
symptoms.
2. The prodromal period (前趨症狀期): characterized by the
appearance of the first mild signs and symptoms.
3. The period of illness (生病期): the disease is at its height
and all disease signs and symptoms are present.
4. The period of decline (衰退期): the signs and symptoms
decrease. Patients’ vulnerable to secondary infection
5. The period of convalescence (康復期): the body returns to
its prediseased state, and health is restored.
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The Stages of a Disease
1 2
3
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4
5
Figure 14.5
The Spread of Infection
Reservoirs of Infection
 Reservoirs of infection are continual sources of infection.
 Human reservoirs (感染源) — AIDS, gonorrhea (淋病),
diphtheria (白喉), typhoid fever (傷寒)
 Carriers may have inapparent infections or latent diseases.
 Animal reservoirs — Rabies (狂犬病), Lyme disease (萊姆病)
 Some zoonoses (人畜共通傳染病) may be transmitted to
humans. Table 14.2
 Nonliving reservoirs — Botulism (肉毒桿菌中毒), tetanus (破
傷風), cholera (霍亂), trichinosis (旋毛蟲病)
 Soil
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Table 14.2 Selected Zoonoses (1 of 2)
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Table 14.2 Selected Zoonoses (2 of 2)
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Transmission of Disease
 Contact transmission:
 Direct contact transmission: requires close association
between the infected and a susceptible host.
 Indirect contact transmission: spreads to a host by a
nonliving object called a fomites (病媒).
 Droplet transmission: transmission via airborne droplets
less than 1 meter.
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Figure 14.6 Contact transmission.
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 Vehicle transmission: Transmission by an inanimate
reservoir.
 Waterborne
 Foodborne
 Airborne
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Figure 14.7 Vehicle transmission.
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 Vectors: Arthropods are the most important group of
disease vectors.
 Mechanical transmission: the passive transport of the
pathogens on the insect’s feet or other body parts.
Arthropod carries pathogen on feet.
 Biological transmission: an active process and is more
complex. The arthropod bites an infected person or
animal and ingests some of the infected blood. Pathogen
reproduces in vector.
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Figure 14.8 Mechanical transmission.
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Table 14.3 Representative Arthropod Vectors and the Diseases They Transmit
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Healthcare-Acquired Infections (HAIs) 院內感染
 HAIs traditionally were
called nosocomial
infections.
 HAIs result from:
 Microorganisms in the
hospital environment
 Weakened status of the
host
 Chain of transmission in a
hospital
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Figure 14.9 Healthcare-associated infections.
 The rate of HAIs has increased 36% during the last 20
years.
 In US, about 2 million people per year contact HAIs,
and nearly 20,000 die as a result.
 HAI represent the 8th leading cause of death in the
US.
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Microorganisms in the Hospital
Gram+
cocci
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Compromised Host (易感染寄主)
 One whose resistance to infection is impaired by
disease, therapy, or burns.
 Two principal conditions:
(1) by broken skin or mucous membrane
(2) a suppressed immune system
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Relative Frequency of HAIs
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Table 14.5下半
Table 14.5 Principal Sites of Healthcare-Associated Infections
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Control of Healthcare-Associated Infections
 Reduce the number of pathogens to which patients are
exposed.
 Handwashing
 Disinfecting tubs used to bathe patients
 Cleaning instruments scrupulously
 Using disposable bandages and intubation
 Infection control committees
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Emerging Infectious Diseases (EIDs)
新興感染性疾病
 Diseases that are new, increasing in incidence, or
showing a potential to increase in the near future.
 Contributing factors:
 New strains resulted from genetic recombination. Ex: E.
coli O157, avian influenza (H5N1)
 A new serovar resulted from changes in or the evolution
of existing microorganisms. Ex: V. cholerae O139
 Inappropriate use of antibiotics and pesticides. Ex:
Antibiotic resistant strains, insects, ticks
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 Changes in weather patterns. Ex: Hantavirus pulmonary
syndrome (HPS, 漢他病毒肺症候群), malaria
 Modern transportation. Ex: Chikungunya (dengue), West
Nile virus (travelers)
 Ecological disaster, war, and expanding human
settlement. Ex: Coccidioidomycosis (球黴菌症)
 Animal control measures. Ex: Lyme disease
 Public Health failure. Ex: Diphtheria epidemic in Soviet
Union in the 1990s
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Table 14.6 Emerging Infectious Diseases (1 of 2)
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Table 14.6 Emerging Infectious Diseases (2 of 2)
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Epidemiology
 Epidemiology: the study
of where and when
diseases occur and how
they are transmitted in
populations.
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Figure 14.10 a, b
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Figure 14.10 c
 Modern epidemiology began in the mid-1800s with
three famous investigations.
John Snow
1848-1849
Mapped the occurrence of cholera in
London
Showed that hand washing decreased
Ignaz Semmelweis 1846-1848 the incidence of puerperal sepsis (產
後敗血症)
Florence
Nightingale
1858
Showed that improved sanitation
decreased the incidence of epidemic
typhus
 Provide the examples of descriptive, analytical and
experimental research model in epidemiology.
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 Three basic types of investigations in epidemiology:
Descriptive
epidemiology
Collection and analysis of data
Snow
Analytical
epidemiology
Analyzes a particular disease to
determine its probable cause
Nightingale
Experimental
epidemiology
Involves a hypothesis and controlled
Semmelweis
experiments
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Centers for Disease Control and Prevention
(CDC, 疾病控制與預防中心)
 Collects and analyzes epidemiological information in
the United States. http://www.cdc.gov
(行政院衛生福利部疾病管制署 www.cdc.gov.tw)
 Publishes Morbidity and Mortality Weekly Report
(MMWR). MMWR contains data on:
 Morbidity (罹病): Incidence of a specific notifiable disease.
 Mortality (死亡): Deaths from notifiable diseases.
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 Morbidity rate: Number of people affected in relation to
the total population in a given time period.
 Mortality rate: Number of deaths from a disease in
relation to the population in a given time.
 Notifiable infectious diseases: diseases in which
physicians are required to report occurrence.
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Figure 14.11 Nationally notifiable diseases, 2013.
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