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– 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.
19.2 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.
• Breaching the host barriers can establish infection
and disease.
– Invasiveness is the ability of a pathogen to penetrate
tissues and spread.
– Many pathogens use phagocytosis by body cells to
enter cells or pass through 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.
– 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.
19.3 Infectious Disease Epidemiology
• Epidemiologists often have to identify the reservoir of
an infectious disease.
– Reservoirs are places in the environment where a
pathogen can be found.
– Carriers have recovered from the disease but
continue to shed the disease agents.
• 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.
• 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 associated with hospitals.
• 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.