Medical Microbiology

                         Medical Microbiology

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General Aspects of Medical
F. H. Kayser
Infectious diseases are caused by subcellular infectious entities (prions,
viruses), prokaryotic bacteria, eukaryotic fungi and protozoans, metazoan animals,
such as parasitic worms (helminths), and some arthropods. Definitive
proof that one of these factors is the cause of a given infection is demonstrated
by fulfillment of the three Henle-Koch postulates. For technical reasons,
a number of infections cannot fulfill the postulates in their strictest
sense as formulated by R. Koch, in these cases a modified form of the postulates
is applied. 

The History of Infectious Diseases
The Past
Infectious diseases have been known for thousands of years, although accurate
information on their etiology has only been available for about a century.
In the medical teachings of Hippocrates, the cause of infections occurring frequently
in a certain locality or during a certain period (epidemics)was sought
in “changes” in the air according to the theory of miasmas. This concept, still
reflected in terms such as “swamp fever” or “malaria,” was the predominant
academic opinion until the end of the 19th century, despite the fact that the
Dutch cloth merchant A. van Leeuwenhoek had seen and described bacteria
as early as the 17th century, using a microscope he built himself with a single
convex lens and a very short focal length. At the time, general acceptance of
the notion of “spontaneous generation”—creation of life from dead organic
material—stood in the way of implicating the bacteria found in the corpses
of infection victims as the cause of the deadly diseases. It was not until Pasteur
disproved the doctrine of spontaneous generation in the second half of
the 19th century that a new way of thinking became possible. By the end of
that century, microorganisms had been identified as the causal agents in
many familiar diseases by applying the Henle-Koch postulates formulated
by R. Koch in 1890.

The History of Infectious Diseases 3
The Henle–Koch Postulates

The postulates can be freely formulated as follows:

  • The microorganism must be found under conditions corresponding to the
    pathological changes and clinical course of the disease in question.
  •  It must be possible to cause an identical (human) or similar (animal) disease
    with pure cultures of the pathogen.
  • The pathogen must not occur within the framework of other diseases as
    an “accidental parasite.”

These postulates are still used today to confirm the cause of an infectious disease.
However, the fact that these conditions are not met does not necessarily
exclude a contribution to disease etiology by a pathogen found in context. In
particular, many infections caused by subcellular entities do not fulfill the
postulates in their classic form.

The Present
The frequency and deadliness of infectious diseases throughout thousands of
years of human history have kept them at the focus of medical science. The
development of effective preventive and therapeutic measures in recent decades
has diminished, and sometimes eliminated entirely, the grim epidemics
of smallpox, plague, spotted fever, diphtheria, and other such contagions. Today we
have specific drug treatments for many infectious diseases. As a result
of these developments, the attention of medical researchers was diverted to
other fields: it seemed we had tamed the infectious diseases. Recent years
have proved this assumption false. Previously unknown pathogens causing
new diseases are being found and familiar organisms have demonstrated
an ability to evolve new forms and reassert themselves. The origins of this
reversal are many and complex: human behavior has changed, particularly
in terms of mobility and nutrition. Further contributory factors were the introduction
of invasive and aggressive medical therapies, neglect of established
methods of infection control, and, of course, the ability of pathogens
to make full use of their specific genetic variability to adapt to changing conditions.
The upshot is that physicians in particular, as well as other medical
professionals and staff, urgently require a basic knowledge of the pathogens
involved and the genesis of infectious diseases if they are to respond effectively
to this dynamism in the field of infectiology. The aim of this textbook is
to impart these essentials to them.
Table 1.1 provides an overview of the causes of human infectious diseases.





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