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Page 32
It is not known whether these organisms belong to the bacteria or
protozoa, and naturally nothing is known as to their form, size and
structure. Up to the present about twenty diseases are known to be due
to a filterable virus, and among these are some of the most important
for animals and for man. Among the human diseases, yellow fever,
poliomyelitis, and dengue are so produced; of the animal diseases in
addition to foot and mouth disease, pleuropneumonia, cattle plague,
African horse sickness, several diseases of fowls and the mosaic
disease of the tobacco plant have all been shown to be due to a
filterable virus. Of these organisms the largest is that which
produces pleuropneumonia in cattle, and this alone has been
cultivated. It gives a slight opacity to the culture fluids, and when
magnified two thousand diameters appears as a minute spiral or round
or stellate organism having a variety of forms. Its size is such that
it passes the coarse, but is held back by the finer, filters and it is
possible that this does not belong to the same class with the
others.[1] The diseases produced by the filterable viruses taken as a
class show much similarity. They run an acute course, are severe, and
the immunity produced by the attack endures for a long time.
Considered in its biological relations, infection is the adaptation of
an organism to the environment which the body of the host offers. It
is rather singular that variations in organisms represented by such
adaptation do not more frequently arise, in which case new diseases
would frequently occur. It cannot be denied that new diseases appear,
but there is no certain evidence that they do, and there is equally no
evidence that diseases disappear. From the meagre descriptions of
diseases, usually of the epidemic type, which have come down to us
from the past, it is difficult to recognize many of the diseases
described. The single diseases are recognized by comparing the causes,
the lesions and the symptoms with those of other diseases, and new
diseases are constantly being separated off from other diseases having
more or less common features. Many new diseases have been recognized
and named, but it is always more than probable that previously they
were confounded with other diseases. Smallpox is such a characteristic
disease that one would think it would have been recognized as an
entity from the beginning, but although the description of some of the
epidemics in remote times conform more or less to the disease as we
know it, the first accurate description is in the eighth century by
the Arabian physician Rhazes. Cerebro-spinal meningitis was not
recognized as a separate disease until 1803, diphtheria not until
1826, and the separation between typhoid and typhus fever was not made
before 1840. Nor is it sure that any diseases have disappeared,
although there seems to have been a change in the character of many.
It is difficult to reconcile leprosy as it appears now with the
universal horror felt towards it, due to the persistence of the old
traditions. It is possible, however, that the disease has not changed
its character, but that such diseases as smallpox, syphilis, and
certain forms of tuberculosis were formerly confounded with leprosy,
thus giving a false idea of its prevalence.
In certain cases the adaptation of the organism is for a narrow
environment; for example, the parasitism may extend to a simple
species only, in others the adaptation may extend to a number of
genera. In certain cases the adaptation is mutual, extending to both
parasite and host and resulting in symbiosis, and this condition may
be advantageous for both. Certain of the protozoa harbor within them
cells of alg� utilizing to their own advantage the green chlorophil of
the alg� in obtaining energy from sunlight and in turn giving
sustenance to the alg�. Although the alg� are useful guests, when they
become too numerous the protozoan devours them. It is evident that
symbiosis is the most favorable condition for the existence of the
parasite, and an injurious action exerted by the parasite on the host
unfavorable. The death of the host is an unfortunate incident from the
parasite's point of view in that it is deprived of habitation and food
supply, being placed in the same unfortunate situation as may befall a
social parasite by the death of his host.
FOOTNOTE:
[1] Flexner has recently succeeded in isolating and cultivating the
organism of poliomyelitis, but the organism is so small that its
classification is not possible.
CHAPTER VII
THE NATURE OF INFECTION.--THE INVASION OF THE BODY FROM ITS
SURFACES.--THE PROTECTION OF THESE SURFACES.--CAN BACTERIA PASS
THROUGH AN UNINJURED SURFACE.--INFECTION FROM WOUNDS.--THE WOUNDS IN
MODERN WARFARE LESS PRONE TO INFECTION.--THE RELATION OF TETANUS TO
WOUNDS CAUSED BY THE TOY PISTOL.--THE PRIMARY FOCUS OR ATRIUM OF
INFECTION.--THE DISSEMINATION OF BACTERIA IN THE BODY.--THE DIFFERENT
DEGREES OF RESISTANCE TO BACTERIA SHOWN BY THE VARIOUS ORGANS.--MODE
OF ACTION OF BACTERIA.--TOXIN PRODUCTION.--THE RESISTANCE OF THE BODY
TO BACTERIA.--CONFLICT BETWEEN PARASITE AND HOST.--ON BOTH SIDES MEANS
OF OFFENSE AND DEFENSE.--PHAGOCYTOSIS.--THE DESTRUCTION OF BACTERIA BY
THE BLOOD.--THE TOXIC BACTERIAL DISEASES.--TOXIN AND
ANTITOXIN.--IMMUNITY.--THE THEORY OF EHRLICH.
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