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CHAPTER
14
Infection, Infectious Diseases,
and Epidemiology
Chapter Outline
Symbiotic Relationships Between Microbes and Their Hosts (pp. 406–410)
Types of Symbiosis
Normal Microbiota in Hosts
How Normal Microbiota Become Opportunistic Pathogens
Reservoirs of Infectious Diseases of Humans (pp. 410–411)
Animal Reservoirs
Human Carriers
Nonliving Reservoirs
The Invasion and Establishment of Microbes in Hosts: Infection (pp. 411–414)
Exposure to Microbes: Contamination and Infection
Portals of Entry
The Role of Adhesion in Infection
The Nature of Infectious Disease (pp. 414–421)
Manifestations of Disease: Symptoms, Signs, and Syndromes
Causation of Disease: Etiology
Virulence Factors of Infectious Agents
The Stages of Infectious Diseases
The Movement of Pathogens Out of Hosts: Portals of Exit (pp. 421–422)
Modes of Infectious Disease Transmission (pp. 422–424)
Contact Transmission
Vehicle Transmission
Vector Transmission
Classification of Infectious Diseases (pp. 424–425)
Epidemiology of Infectious Diseases (pp. 425–433)
Frequency of Disease
Epidemiological Studies
Hospital Epidemiology: Nosocomial Infections
Epidemiology and Public Health
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Chapter Summary
This chapter examines the relationships between pathogenic microbes and their hosts, and
concludes with a discussion of epidemiology.
Symbiotic Relationships Between Microbes and Their Hosts
(pp. 406–409)
Symbiosis means “to live together.”
Types of Symbiosis
Microbes live with their hosts in symbiotic relationships. Types of symbiosis include
mutualism, in which both members benefit; commensalism, in which one member benefits
while the other is relatively unaffected; and parasitism, in which a parasite benefits while the
host is harmed. Any parasite that causes disease is called a pathogen.
Normal Microbiota in Hosts
Organisms called normal microbiota live in and on the body, normally without causing disease. Most are commensal. Some of these microbes are resident from birth to death, whereas
others are transient. The womb is an axenic environment, free of microbes. At birth, microorganisms come into contact with the baby’s body, and the first breath and food introduce
others.
How Normal Microbiota Become Opportunistic Pathogens
Opportunistic pathogens cause disease when normal microbiota are introduced into an abnormal area of the body; when the immune system is suppressed; and when changes in the
body, such as consumption of antibiotics, disrupt normal microbial antagonism.
Reservoirs of Infectious Diseases of Humans (pp. 410–411)
If pathogens are to enter new hosts, they must survive in some site from which they can infect
new hosts. These sites where pathogens are maintained as a source of infection are called
reservoirs of infection. They include animal hosts, human carriers, and nonliving reservoirs.
Animal Reservoirs
Diseases that spread naturally from animals to humans are called zoonoses. Disease may
spread through direct contact with the animal or its waste products, by consuming the animal,
or through an arthropod vector. When zoonotic diseases have large animal reservoirs, it can
be difficult to control the spread to humans.
Human Carriers
Humans with active infections are important reservoirs of human disease. Humans may be
asymptomatic carriers of infection. Some carriers eventually develop the disease, whereas
others remain a continued source of infection without ever becoming sick. Presumably, many
such healthy carriers have defensive systems that protect them from illness.
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Nonliving Reservoirs
Nonliving reservoirs of infection include soil, water, food, and objects.
The Invasion and Establishment of Microbes in Hosts: Infection
(pp. 411–414)
Exposure to Microbes: Contamination and Infection
Microbial contamination refers to the mere presence of microbes in or on the body. Microbial contaminants include relatively harmless resident and transient members of the normal microbiota, as well as pathogens. Successful invasion of the body by a pathogen is called an
infection. An infection may or may not result in disease.
Portals of Entry
The entry sites of pathogens into the human body are called portals of entry. They include
the natural openings of the skin, such as hair follicles and sweat glands, as well as abraded
skin, cuts, and scrapes. Other portals of entry are the mucous membranes lining the respiratory, gastrointestinal, urinary, and reproductive tracts as well as the conjunctiva covering the
surface of the eye. In about 2% of pregnancies, the placenta becomes a portal of entry for
infection of the fetus. The parenteral route, by which microbes are directly deposited into
deep tissues via stab wounds, surgery, and so on, is a means by which portals of entry can be
circumvented.
The Role of Adhesion in Infection
Adhesion (or attachment) is the process by which pathogens attach to cells. It is accomplished via a variety of structures or chemical receptors called adhesion factors (adhesins or
ligands). The interaction between ligand and receptor on the host cell can determine pathogen
specificity for host cells. Some bacteria have lost the ability to make adhesion factors and
have thereby lost their virulence. Some pathogens interact to produce a sticky biofilm that
adheres to a surface in place of direct bacterial attachment to cells.
The Nature of Infectious Disease (pp. 414–421)
Disease, also known as morbidity, is a sufficiently adverse condition interfering with normal
functioning of the body, or any change from a state of health. Notice that a person can be
infected, such as with HIV, but not yet have the disease.
Manifestations of Disease: Symptoms, Signs, and Syndromes
Diseases manifest in different ways. Symptoms are subjective characteristics of a disease that
only the patient can feel, such as pain. In contrast, signs are objectively observable and often
measurable by others, such as a high temperature. A syndrome is a group of symptoms and
signs that collectively characterizes a particular disease or abnormal condition. For example,
malaise, loss of helper T cells, diarrhea, weight loss, pneumonia, toxoplasmosis, and tuberculosis characterize AIDS. Asymptomatic or subclinical infections by definition lack symptoms, but may have detectable signs.
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Causation of Disease: Etiology
There are many categories of disease (hereditary, congenital, neoplastic, and so forth): in this
text we focus on infectious disease. The study of the cause of a disease is called etiology. In
the 19th century, Pasteur, Koch, and other scientists proposed the germ theory of disease,
which states that disease is caused by infections of pathogenic microorganisms. Koch’s postulates are the cornerstone of infectious disease etiology. To prove that a given infectious
agent causes a given disease, a scientist must satisfy all four postulates:
1. The suspected pathogen must be present in every case of the disease.
2. The pathogen must be isolated and grown in pure culture.
3. The cultured pathogen must cause the disease when introduced into a healthy host.
4. The same pathogen must be reisolated from the diseased host.
Certain circumstances can make the use of Koch’s postulates difficult or even impossible.
Some microbes have never been grown in the laboratory. Some diseases are the result of the
combined effects of microbes and environment or of multiple pathogens. Ethical considerations prevent completion of the third step, as when humans are the only susceptible host. Certain syndromes, such as pneumonia, do not have a single causative agent.
Virulence Factors of Infectious Agents
The ability of a microorganism to cause disease is called pathogenicity, and the degree of
pathogenicity is called virulence. Pathogens have a variety of traits that enable them to cause
disease; collectively, these are called virulence factors. Virulence factors include:
 Extracellular enzymes that enable them to dissolve structural chemicals in the body
 The ability to produce toxins that can harm tissues, trigger damaging immune responses,
or result in toxemia, the presence of toxins in the bloodstream
Exotoxins are secreted by pathogens into their environment; they include cytotoxins,
enterotoxins, and neurotoxins. The body protects itself with antitoxins, which are antibodies the host forms to bind with and destroy toxins.
Endotoxin, also known as lipid A, is released from the outer membrane of dead and dying Gram-negative bacteria and can cause systemic shock and other fatal effects.
 Some pathogens produce antiphagocytic factors, such as hyaluronic acid capsules and
chemicals, which allow them to survive inside of macrophages
The Stages of Infectious Diseases
Following infection, a sequence of events called the disease process occurs. In many cases,
this process consists of five stages:
1. The incubation period is the time between an infection and occurrence of the first
symptoms.
2. The prodromal period is a short time preceding illness of generalized, mild symptoms,
such as malaise.
3. Illness is the most severe stage, when signs and symptoms are most evident.
4. Decline is the stage during which the patient’s body gradually returns to normal as the
immune response and any medical treatments vanquish the pathogen.
5. During convalescence, the patient recovers and tissues are repaired.
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A patient may be infectious during every stage of disease.
The Movement of Pathogens Out of Hosts: Portals of Exit
(pp. 421–422)
Just as infections occur through portals of entry, so pathogens must leave infected patients
through portals of exit in order to infect others. These include bodily secretions, blood, and
bodily wastes as well as some of the portals of entry discussed earlier.
Modes of Infectious Disease Transmission (pp. 422–424)
Disease can be transmitted by numerous modes, which are somewhat arbitrarily categorized
into three groups: contact transmission, vehicle transmission, and vector transmission.
Contact Transmission
Direct contact transmission of infectious disease involves person-to-person spread by
bodily contact. Indirect contact transmission occurs when pathogens are spread from one
host to another via fomites, inanimate objects inadvertently used to transfer pathogens. These
objects include needles, tissues, toys, coins, and so forth. Droplet transmission is a third
type of contact transmission: Pathogens can be transmitted via droplet nuclei that exit the
body during exhalation, sneezing, and coughing and travel less than 1 meter.
Vehicle Transmission
Vehicle transmission is the spread of pathogens via air, drinking water, and food as well as
bodily fluids handled outside the body. Airborne transmission involves the spread of pathogens farther than 1 meter to the respiratory mucous membranes of a new host via an aerosol,
a cloud of small droplets and solid particles suspended in the air. Aerosols can also be generated by activities such air conditioning or sweeping. Waterborne transmission is important
in the spread of many gastrointestinal pathogens, including giardiasis, and cholera. Fecaloral infection is a major source of disease in the world, as certain worms and enteroviruses
shed in the feces enter the gastrointestinal system. Foodborne transmission involves pathogens in and on foods that are inadequately processed, undercooked, or poorly refrigerated.
Bodily fluid transmission can occur when blood, urine, and other fluids are handled outside
the body.
Vector Transmission
Vectors are animals such as arthropods that transmit diseases from one host to another.
Biological vectors not only transmit pathogens but also serve as hosts for the multiplication
of a pathogen during some stage of its life cycle. Mechanical vectors are not required as
hosts by the pathogens they transmit; such vectors only passively carry pathogens to new
hosts on their feet or other body parts.
Classification of Infectious Diseases (pp. 424–425)
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Infectious diseases can be classified in a number of ways, each of which has its own advantages. Diseases can be classified by the body system affected, but many diseases affect
more than one system. Alternatively disease can be classified by taxonomic group. When
grouped by time course and severity, diseases may be described as acute, subacute, chronic,
or latent. An acute disease develops rapidly but lasts only a short time, either resolving or
causing death of the host. The common cold is an example A chronic disease develops slowly but is continual or recurrent. An example is tuberculosis. Subacute diseases have durations
and severities that lie somewhere between acute and chronic. Latent diseases are those in
which a pathogen remains inactive for a long period of time before producing signs and
symptoms. An example is herpes.
Infectious diseases are also classified as communicable, if they come from another infected host, or as contagious, if they are easily transmitted between hosts. Measles is a communicable disease. Noncommunicable infectious diseases arise from outside hosts or from
normal microbiota. Acne and tooth decay are examples.
Epidemiology of Infectious Diseases (pp. 425–433)
Epidemiology is the study of the location, course, and transmission of diseases within
populations.
Frequency of Disease
Epidemiologists track the incidence (that is, the number of new cases of a disease in a given
area or population in a given period of time) as well as the prevalence (the total number of
cases in a given area or population in a given period of time). The occurrence of infectious
disease can also be considered in terms of a combination of frequency and geographic distribution. A disease that normally occurs continually at a relatively stable incidence within a
given population or area is said to be endemic to that population or area. A disease is considered sporadic when only a few scattered cases occur. Whenever a disease occurs at a greater
frequency than normal for a population or area, it is said to be epidemic. A pandemic is an
epidemic occurring simultaneously on more than one continent.
Epidemiological Studies
Descriptive epidemiology is the careful recording of data concerning a disease; it often includes detection of the index case, the first case of the disease in a given area or population.
Analytical epidemiology seeks to determine the probable cause of a disease, its mode of
transmission, and possible means of prevention. Experimental epidemiology involves
testing a hypothesis resulting from analytical studies concerning the cause of a disease.
Hospital Epidemiology: Nosocomial Infections
Nosocomial infections and nosocomial diseases are acquired by patients or staff in health
care facilities. They may be exogenous (acquired from the health care environment, such as
in air-conditioning systems or on bed rails), endogenous (derived from normal microbiota
that become opportunistic while the patient is in the health care setting), or iatrogenic (induced by treatment, such as with antibiotics; or medical procedures, such as surgery). Factors
contributing to nosocomial infections include the numerous pathogens present in health care
settings and the weakened immune systems of patients. Aggressive control measures can reduce the incidence of nosocomial infections.
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Epidemiology and Public Health
Public health organizations from local to global levels use epidemiological data to promulgate rules and standards to promote public health. Potable water, that is, water fit to drink is
vital. They also work to regulate food safety standards, to prevent disease by controlling vectors and animal reservoirs, to establish immunization schedules, and to educate people so they
can make healthful choices concerning the prevention of disease.
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