Chapter 113. Introduction to Infectious Diseases:
Host–Pathogen Interactions
(Part 6)

The microbiology laboratory must be an ally in the diagnostic endeavor.
Astute laboratory personnel will suggest optimal culture and transport conditions
or alternative tests to facilitate diagnosis. If informed about specific potential
pathogens, an alert laboratory staff will allow sufficient time for these organisms
to become evident in culture, even when the organisms are present in small
numbers or are slow-growing. The parasitology technician who is attuned to the
specific diagnostic considerations relevant to a particular case may be able to
detect the rare, otherwise-elusive egg or cyst in a stool specimen. In cases where a
diagnosis appears difficult, serum should be stored during the early acute phase of
the illness so that a diagnostic rise in titer of antibody to a specific pathogen can be
detected later. Bacterial and fungal antigens can sometimes be detected in body
fluids, even when cultures are negative or are rendered sterile by antibiotic
therapy. Techniques such as the polymerase chain reaction allow the amplification
of specific DNA sequences so that minute quantities of foreign nucleic acids can
be recognized in host specimens.
Infectious Diseases: Treatment
Optimal therapy for infectious diseases requires a broad knowledge of
medicine and careful clinical judgment. Life-threatening infections such as
bacterial meningitis or sepsis, viral encephalitis, or falciparum malaria must be
treated immediately, often before a specific causative organism is identified.
Antimicrobial agents must be chosen empirically and must be active against the
range of potential infectious agents consistent with the clinical scenario. In
contrast, good clinical judgment sometimes dictates withholding of antimicrobial
drugs in a self-limited process or until a specific diagnosis is made. The dictum
primum non nocere should be adhered to, and it should be remembered that all
antimicrobial agents carry a risk (and a cost) to the patient. Direct toxicity may be
encountered—e.g., ototoxicity due to aminoglycosides, lipodystrophy due to

bone marrow transplant recipients. There is a strong need for well-designed
clinical trials to evaluate each new interventional modality.
Perspective
The genetic simplicity of many infectious agents allows them to undergo
rapid evolution and to develop selective advantages that result in constant
variation in the clinical manifestations of infection. Moreover, changes in the
environment and the host can predispose new populations to a particular infection.
The dramatic march of West Nile virus from a single focus in New York City in
1999 to locations throughout the North American continent by the summer of
2002 caused widespread alarm, illustrating the fear that new plagues induce in the
human psyche. The intentional release of deadly spores of Bacillus anthracis via
the U.S. Postal Service awakened many from a sense of complacency regarding
biologic weapons.
"The terror of the unknown is seldom better displayed than by the response
of a population to the appearance of an epidemic, particularly when the epidemic
strikes without apparent cause." Edward H. Kass made this statement in 1977 in
reference to the newly discovered Legionnaire's disease, but it could apply equally
to SARS, H5N1 (avian) influenza, or any other new and mysterious disease. The
potential for infectious agents to emerge in novel and unexpected ways requires
that physicians and public health officials be knowledgeable, vigilant, and open-
minded in their approach to unexplained illness. The emergence of antimicrobial-
resistant pathogens (e.g., enterococci that are resistant to all known antimicrobial
agents and cause infections that are essentially untreatable) has led some to
conclude that we are entering the "postantibiotic era." Others have held to the
perception that infectious diseases no longer represent as serious a concern to
world health as they once did. The progress that science, medicine, and society as
a whole have made in combating these maladies is impressive, and it is ironic that,
as we stand on the threshold of an understanding of the most basic biology of the
microbe, infectious diseases are posing renewed problems. We are threatened by
the appearance of new diseases such as SARS, hepatitis C, and Ebola virus

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