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U N I T 4
Infection and Immunity
Epidemiology of Infectious
Diseases
Epidemiologists working in infectious disease study the
factors, events, and circumstances that influence the trans-
mission of infectious diseases among humans. The ulti-
mate goal of epidemiologists is to devise strategies that
interrupt or eliminate the spread of infectious agents. To
accomplish this, they classify infectious diseases accord-
ing to incidence, portal of entry and source, symptoms,
disease course, site of infection, and virulence factors so
that potential outbreaks may be predicted and averted or
appropriately treated.
Mechanisms ofTransmission
The outcomes of infections depend on the ability of
microbes to breach host barriers and colonize and
damage host tissues. Host barriers to infection prevent
the microbes from entering the body and assist innate
and adaptive immune defenses in eliminating the agent
(see Chapter 15).
Portal of Entry
Microbes can enter the host by direct contact, ingestion,
and inhalation. The portal of entry does not dictate the
site of infection. Ingested pathogens may penetrate the
intestinal mucosa, disseminate through the circulatory
system, and cause diseases in other organs such as the
lung or liver. Whatever the mechanisms of entry, the
transmission of infectious agents is directly related to
the number of infectious agents absorbed by the host.
Penetration.
Any disruption in the integrity of the
body’s surface barrier—skin or mucous membranes—
is a potential site for invasion of microorganisms. The
break may be the result of an accidental injury causing
abrasions, burns, or penetrating wounds; medical pro-
cedures such as surgery or catheterization; or a primary
infectious process such as chickenpox or impetigo that
produces surface lesions. Direct inoculation from intra-
venous drug use or an animal or arthropod bite also can
occur.
Direct Contact.
Some pathogens are transmitted
directly from infected tissue or secretions to exposed,
intact mucous membranes. This is especially true of
certain sexually transmitted infections (STIs) such as
gonorrhea, syphilis, chlamydia, and genital herpes, for
which exposure of uninfected membranes to pathogens
occurs during intimate contact (see Chapter 41).
The transmission of STIs is not limited to sexual con-
tact.
Vertical transmission
of these agents, from mother
to child, can occur across the placenta or during birth
when the mucous membranes of the child come in con-
tact with infected vaginal secretions of the mother. When
an infectious disease is transmitted from mother to child
during gestation or birth, it is classified as a
congeni-
tal infection.
The most frequently observed congenital
infections include toxoplasmosis (caused by the para-
site
Toxoplasma gondii
), syphilis, rubella, cytomegalo-
virus infection, and herpes simplex virus infections (the
so-called TORCH infections, discussed in Chapter 6);
varicella-zoster (chickenpox); parvovirus B19; group
B streptococci (
Streptococcus agalactiae
); and HIV. Of
these, cytomegalovirus is by far the most common cause
of congenital infection in the United States, affecting
nearly 1% of all newborns. However, with more than
500,000 babies contracting HIV from their mother and
with a 25% to 30% chance of vertical transmission
without appropriate antiretroviral treatment, HIV is
rapidly gaining in stature as a congenitally transmitted
infection (see Chapter 16).
Ingestion.
The entry of pathogenic microorganisms or
their toxins through the oral cavity and gastrointesti-
nal tract represents one of the more efficient means of
disease transmission in humans. Many bacterial, viral,
and parasitic infections, including cholera, typhoid
fever, dysentery (amebic and bacillary), food poisoning,
traveler’s diarrhea, cryptosporidiosis, and hepatitis A,
are initiated through the ingestion of contaminated food
and water. This mechanism of transmission necessitates
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Bacteria are autonomously replicating unicellular
organisms known as prokaryotes because
they lack an organized nucleus. Most bacteria
produce a cell wall that is produced only by
prokaryotes and is therefore an attractive target
for antibacterial therapy.
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The Rickettsiaceae, Anaplasmataceae,
Chlamydiaceae, and Coxiella are obligate
intracellular pathogens, like the viruses, but
produce a rigid peptidoglycan cell wall, reproduce
asexually by cellular division, and contain RNA
and DNA, similar to the bacteria. Chlamydial
diseases include sexually transmitted genital
infections, neonatal and adult conjunctivitis, and
infant pneumonia.
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Fungi are free-living, eukaryotic saprophytes,
some of which are members of the normal human
microflora. Fortunately, few fungi are capable of
causing diseases in humans, and most of these
are incidental, self-limited infections of skin and
subcutaneous tissue. Despite their normally
harmless nature, fungi can cause life-threatening
opportunistic diseases when host defense
capabilities have been disabled.
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Parasites (protozoa, helminths, and arthropods)
are members of the animal kingdom that infect or
colonize other animals, which then transmit them
to humans. In some cases, they directly infect the
human host.
SUMMARY CONCEPTS
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