Download Chapter 13: Infection and Disease

Chapter 13
Infection and Disease
13.1 The Host–Microbe Relationship
• The Human Body Maintains a Symbiosis with Microbes
• Infection refers to:
• The relationship between the host and microbe
• The competition for supremacy between them
• If the host loses the competition, disease occurs
• Microbiota are microbes that reside in the body without causing disease
(in symbiosis)
• In mutualism, both host and microbe benefit
• In commensalism, the microbe benefits and the host is unaffected
• Pathogens Differ in Their Ability to Cause Disease
• In parasitism, pathogens cause damage and disease in the host
• Pathogenicity refers to a microbe’s ability to enter a host and cause disease
• Virulence is the degree of pathogenicity
• Pathogenicity islands refers to gene clusters responsible for virulence
• Several Events Must Occur for Disease to Develop in the Host
• An exogenous infection occurs if a pathogen breaches the host’s external
defense and enters sterile tissue
• An endogenous infection occurs if normal microbiota enter sterile tissue
• Opportunistic infections occur when commensals take advantage of a
change in the body’s environment that favors the microbe
• Primary infections occur in otherwise healthy bodies
• Secondary infections occur in a body weakened by a primary infection
• Local diseases are restricted to a single area
• Systemic diseases disseminate to organs and systems
13.2 Establishment of Infection and Disease
• Diseases Progress through a Series of Stages
• Signs, symptoms, and syndromes characterize diseases
• The incubation period is the time between entry of the microbe and
symptom appearance
• The prodromal phase is a time of mild signs or symptoms
• The acme period (climax) is when signs and symptoms are most intense
• As signs and symptoms subside, a period of decline occurs
• The body systems return to normal during the period of convalescence
• Acute diseases develop rapidly, cause severe symptoms, and fade quickly
• Chronic diseases linger for long periods of time, and are slower to develop
and recede
• Pathogen Entry into the Host Depends on Cell Adhesion and the Infectious Dose
• The portal of entry is the route an exogenous pathogen uses to enter the
body
• The infectious dose is the number of microbes entering the body
• Many pathogens have adhesins that allow them to adhere to specific tissues
• Breaching the Host Barriers Can Establish Infection and Disease
• Invasiveness is the ability of a pathogen to penetrate tissues and spread
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Many pathogens use phagocytosis by body cells to enter cells or pass
through defenses
• Successful Invasiveness Requires Pathogens to Have Virulence Factors
• Pathogens have to adapt to a new environment when they enter a host
• Enzymes can help pathogens resist body defenses
• Some staphylococci produce coagulase to form a blood clot that protects them
from phagocytosis
• Others produce streptokinase, which dissolves fibrin clots and allows
dissemination of the bacteria
• Hyaluronidase enhances pathogen penetration through tissues
• Leukocidins disintegrate neutrophils and macrophages
• Hemolysins dissolve red blood cells
• Virulence can be enhanced in biofilms because immune cells cannot reach
bacterial cells
• Toxigenicity is the ability of pathogens to produce toxins
• Toxemia is the presence of toxins in the blood
• Exotoxins are proteins produced during bacterial metabolism
• Neurotoxins act on the nervous system
• Enterotoxins act on the gastrointestinal tract
• Antitoxins are produced by the host body and neutralize toxins
• Toxoids are toxins whose toxicity has been destroyed but still elicit an immune
response
• Endotoxins are released upon disintegration of gram-negative cells
• They cause blood coagulation
• Endotoxin shock may occur with antibiotic treatment of diseases caused
by gram-negative bacilli
• Pathogens Must Be Able to Leave the Host to Spread Disease
• Pathogens or toxins leave the host through a portal of exit
13.3 Infectious Disease Epidemiology
• Epidemiologists Have Several Terms that Apply to the Infectious Disease Process
• Communicable diseases are contagious
• They are transmittable among hosts in a population
• Noncommunicable diseases are not easily transmitted to another host
• They are acquired directly from the environment
• Infectious Diseases Can Be Transmitted in Several Ways
• Direct contact methods involve close or personal contact with an infected
person
• Indirect contact methods can involve fomites
• Epidemiologists Often Have to Identify the Reservoir of an Infectious Disease
• Reservoirs are ecological niches where microbes live and reproduce
• Carriers have recovered from the disease but continue to shed the disease
agents
• Diseases Are Also Described by How They Occur Within a Population
• An endemic disease is habitually present at a low level in a certain
geographic area
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An epidemic disease occurs in a region in excess of what is normally
found in that population
• An outbreak is a more contained epidemic
• A pandemic is a worldwide epidemic
Nosocomial Infections Are Serious Health Threats within the Health Care System
• Health care-associated infections (HAIs) occur as a result of receiving
treatment for another condition
• Nosocomial infections are HAIs associated with hospitals
• They occur as a result of chains of transmission
Infectious Diseases Continue to Challenge Public Health Organizations
• Globalization means that diseases emerging anywhere in the world can be
spread globally
• Zoonoses are diseases transmitted from other vertebrate animals to
humans
• Disease emergence and reemergence are related to:
• Changes in land use or agriculture practices
• Changes in human demographics
• Poor population health
• Pathogen evolution
• Contamination of food sources and water supplies
• International travel
• Failure of public health systems
• International trade
• Climate change