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CHAPTER

Infection, Infectious Diseases,

and Epidemiology

Chapter Outline

Symbiotic Relationships Between Microbes and Their Hosts (pp. 411–415)

Types of Symbiosis

Microbiome of Humans

How Normal Microbiota Become Opportunistic Pathogens

Reservoirs of Infectious Diseases of Humans (pp. 415–417)

Animal Reservoirs

The Invasion and Establishment of Microbes in Hosts: Infection (pp. 417–419)

Exposure to Microbes: Contamination and Infection

Portals of Entry

The Role of Adhesion in Infection

The Nature of Infectious Disease (pp. 419–427)

Manifestations of Disease: Symptoms, Signs, and Syndromes

Causation of Disease: Etiology

The Movement of Pathogens Out of Hosts: Portals of Exit (p. 427)

Modes of Infectious Disease Transmission (pp. 427–429)

Contact Transmission

Vehicle Transmission

Vector Transmission

Classification of Infectious Diseases (pp. 429–431)

Epidemiology of Infectious Diseases (pp. 431–438)

Frequency of Disease

Instructor’s Manual for Microbiology with Diseases by Body System, 5e

Chapter Summary

This chapter examines how microbes affect our bodies. We first examine the types of relation-

Symbiotic Relationships Between Microbes and Their Hosts

(pp. 411–415)

Symbiosis means “to live together.” There are different types of symbiosis.

Types of Symbiosis

Members of a symbiotic relationship are symbionts. Types of symbiosis include:

 Mutualism, in which both symbionts benefit.

Microbiome of Humans

Many parts of the body are axenic—free of microbes. The body’s microbiome consists of mi-

crobes that colonize the body without normally causing diseases. The microbiome is sometimes

referred to as normal microbiota, normal flora, or indigenous microbiota.

Resident Microbiota

Some of these microbes are resident from birth to death. They are found on the skin, mucous

dead cells without harming the body.

Transient Microbiota

Other microbiota are transient, living in or on the body for a limited time before disappearing.

They are in the same locations as the resident microbiota.

Acquisition of Normal Microbiota

How Normal Microbiota Become Opportunistic Pathogens

Chapter 14 Infection, Infectious Diseases, and Epidemiology

tition. Microbial antagonism occurs when normal microbiota create conditions that make it less

likely that pathogens can become established. Stress can also disrupt normal microbiota.

Reservoirs of Infectious Diseases of Humans (pp. 415–417)

If pathogens are to enter new hosts, they must survive in some site from which they can infect

Animal Reservoirs

Diseases that spread naturally from animals to humans are called zoonoses. Disease may spread

Human Carriers

Humans with active infections are important reservoirs of human disease. Humans may be

Nonliving Reservoirs

Nonliving reservoirs of infection include soil, water, food, and objects.

The Invasion and Establishment of Microbes in Hosts: Infection

(pp. 417–419)

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 microbi-

Portals of Entry

The entry sites of pathogens into the human body are called portals of entry. They include the

Skin

The skin is thick, dry, and dead. Pathogens can gain entry through natural openings of the skin,

Instructor’s Manual for Microbiology with Diseases by Body System, 5e

Mucous Membranes

Major portals of entry are the mucous membranes lining the respiratory, gastrointestinal, uri-

Placenta

The placenta forms an effective barrier the majority of the time. In about 2% of pregnancies, the

placenta becomes a portal of entry for infection of the fetus.

Parenteral Route

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 (like suckers) or chemical receptors called adhesion factors (ligands:

The Nature of Infectious Disease (pp. 419–427)

Though most infectious agents are inhibited by the body’s immune defenses, some are able to

invade the body and cause harm. Disease, also known as morbidity, is an injury sufficient to in-

terfere with normal functioning of the body, or any change from a state of health. Notice that in-

fection and disease are not the same. For example, 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 objective, observable, and often

Causation of Disease: Etiology

There are many categories of disease (hereditary, congenital, neoplastic, etc ): in this text we

focus on infectious disease. The study of the cause of disease is called etiology.

Chapter 14 Infection, Infectious Diseases, and Epidemiology

Using Koch’s Postulates

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. Robert Koch

Exceptions to Koch’s Postulates

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

Virulence Factors of Infectious Agents

The ability of a microorganism to cause disease is called pathogenicity, and the degree of path-

ogenicity is called virulence. Pathogens have a variety of traits that enable them to cause dis-

ease; collectively, these are called virulence factors. Virulence factors include:

 Extracellular enzymes. Pathogens produce extracellular enzymes to dissolve structural chem-

icals in the body. Hyaluronidase and coagulase are examples.

 Toxins. Toxins produced by pathogens harm tissues, trigger damaging immune responses, or

result in toxemia, the presence of toxins in the bloodstream.

 Video Tutor: Some Virulence Factors

Instructor’s Manual for Microbiology with Diseases by Body System, 5e

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 symp-

toms.

The Movement of Pathogens Out of Hosts: Portals of Exit

(p. 427)

Just as infections occur through portals of entry, so pathogens must leave infected patients

Modes of Infectious Disease Transmission (pp. 427–429)

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 an-

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 patho-

gens 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

Vector Transmission

Vectors are arthropods that transmit diseases from one host to another.

Biological vectors not only transmit pathogens but also serve as hosts for the multiplication

Classification of Infectious Diseases (pp. 429–431)

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

Epidemiology of Infectious Diseases (pp. 431–438)

Epidemiology is the study of the timing, location, 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 ar-

Epidemiological Studies

Epidemiologists employ three different approaches to study disease in populations: descriptive,

analytical, and experimental.

Instructor’s Manual for Microbiology with Diseases by Body System, 5e

Analytical epidemiology seeks to determine the probable cause of a disease, its mode of

Hospital Epidemiology: Healthcare-Associated (Nosocomial) Infections

Of special concern are healthcare-associated infections (HAIs, formerly nosocomial infec-

tions). HAIs are acquired by patients or staff in health care facilities, including hospitals, dental

clinics, and doctors’ offices. Healthcare-associated diseases (nosocomial diseases) increase the

cost of medical care and are responsible for about 100,000 deaths annually in the United States.

Types of Healthcare-Associated Infections

HAIs 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 opportunis-

Factors Influencing Healthcare-Associated Infections

Contributing factors of HAIs include the numerous pathogens present in health care settings; the

weakened immune systems of patients; and transmission among staff, visitors, and patients.

Control of Healthcare-Associated Infections

Aggressive control measures can reduce the incidence of HAIs. These include disinfection, anti-

sepsis, effective handwashing, housekeeping, and food handling practices.

Epidemiology and Public Health

The goal of epidemiologists is to reduce disease and improve health in communities.

The Sharing of Data Among Public Health Organizations

Public health organizations from local to global levels gather epidemiological data and submit

The Role of Public Health in Interrupting Disease Transmission

Public health officials seek to limit disease transmission by a number of methods.

 Enforce food and water safety standards.

 Reduce vectors and animal reservoirs.

 Establish and enforce immunization schedules.

Chapter 14 Infection, Infectious Diseases, and Epidemiology

 Locate and treat people exposed to disease.

Public Health Education