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Page 25
The knowledge which has come from the study of infectious disease has
served also to broaden our conception of disease and has created
preventive medicine; it has linked more closely to medicine such
sciences as zo�logy and botany; it has given birth to the sciences of
bacteriology and protozo�logy and in a way has brought all sciences
more closely together. Above all it has made medicine scientific, and
never has knowledge obtained been more quickening and stimulating to
its pursuit.
Although the dimensions of this book forbid much reference to the
historical development of a subject, some mention must still be made
of the development of knowledge of the infectious diseases. It was
early recognized that there were diseases which differed in character
from those generally prevalent; large numbers of people were affected
in the same way; the disease beginning with a few cases gradually
increased in intensity until an acme was reached which prevailed for a
time and the disease gradually disappeared. Such diseases were
attributed to changes in the air, to the influence of planets or to
the action of offended gods. The priests and charlatans who sought to
excuse their inability to treat epidemics successfully were quick to
affirm supernatural causes. Hippocrates (400 B.C.), with whom medicine
may be said to begin, thought such diseases, even then called
epidemics, were caused by the air; he says, "When many individuals are
attacked by a disease at the same time, the cause must be sought in
some agent which is common to all, something which everyone uses, and
that is the air which must contain at this time something injurious."
Aristotle recognized that disease was often conveyed by contact, and
Varro (116-27 B.C.) advanced the idea that disease might be caused by
minute organisms. He says, "Certain minute organisms develop which the
eye cannot see, and which being disseminated in the air enter into the
body by means of the mouth and nostrils and give rise to serious
ailments." In spite of this hypothesis, which has proved to be
correct, the belief became general that epidemics were due to
putrefaction of the air brought about by decaying animal bodies, (this
explaining the frequent association of epidemics and wars,) by
emanations from swamps, by periods of unusual heat, etc.
With the continued study of epidemics the importance of contagion was
recognized; it was found that epidemics differed in character and in
the modes of extension. Some seemed to extend by contact with the
sick, and in others this seemed to play no part; it was further found
impossible in many cases to show evidence of air contamination, and
contamination of the air by putrefactive material did not always
produce disease. Most important was the recognition that single cases
of diseases which often occurred in epidemic form might be present and
no further extension follow; this led to the assumption in epidemics
of the existence of some condition in addition to the cause, and which
made the cause operative. In this way arose the theory of the epidemic
constitution, a supposed peculiar condition of the body due to changes
in the character of the air, or to the climate, or to changes in the
interior of the earth as shown by earthquakes, or to the movements of
planets; in consequence of this peculiar constitution there was a
greater susceptibility to disease, but the direct cause might arise in
the interior of the body or enter the body from without. The character
of the disease which appeared in epidemic form, the "Genius
epidemicus," was determined not by differences in the intrinsic cause,
but by the type of constitution which prevailed at that time. The
first epidemic of cholera which visited Europe in 1830-37 was for the
most part referred to the existence of a peculiar epidemic
constitution for which various causes were assigned. It was only when
the second epidemic of this disease appeared in 1840 that the
existence of some special virus or poison which entered the body was
assumed.
Meanwhile, by the study of the material of disease knowledge was being
slowly acquired which had much bearing on the causes. The first
observations which tended to show that the causes were living were
made by a learned Jesuit, Athanasius, in 1659. He found in milk,
cheese, vinegar, decayed vegetables, and in the blood and secretions
of cases of plague bodies, which he described as tiny worms and which
he thought were due to putrefaction. He studied these objects with the
simple lenses in use at that time, and there is little doubt that he
did see certain of the larger organisms which are present in vinegar,
cheese and decaying vegetables, and it is not impossible that he may
have seen the animal and vegetable cells.
The first description of bacteria with illustrations showing their
forms was given by Loewenhoeck, a linen dealer in Amsterdam in 1675.
The fineness of the linen being determined by the number of threads in
a given area, it is necessary to examine it with a magnifying lens,
and he succeeded in perfecting a simple lens with which objects
smaller than had been seen up to that time became visible. It must be
added that he was probably endowed with very unusual acuteness of
vision. He found in a drop of water, in the fluid in the intestines of
frogs and birds, and in his evacuations, objects of great minuteness
which differed from each other in form and size and in the peculiar
motion which some of them possessed. In the year 1683 he presented to
the Royal Society of London a paper describing a certain minute
organism which he found in the tartar of his teeth. After these
observations of Loewenhoeck became known to the world they quickly
found application in disease, although the author had expressed
himself very cautiously in this regard. The strongest exponent of the
view of a living contagion was Plenciz, 1762, a physician of Vienna,
basing his belief not only on the demonstration of minute organisms by
Loewenhoeck which he was able to verify, but on certain shrewdly
conceived theoretical considerations. He was the first to recognize
the specificity of the epidemic diseases, and argued from this that
each disease must have a specific cause. "Just as a certain plant
comes from the seed of the same plant and not from any plant at will,
so each contagious disease must be propagated from a similar disease
and cannot be the result of any other disease." Further he says, "It
is necessary to assume that during the prevalence of an epidemic the
contagious material undergoes an enormous increase, and this is
compatible only with the assumption that it is a living substance."
But as is so often the case, speculation ran far ahead of the
observations on which it is based. There was a long gap between the
observations of Loewenhoeck and the theories of Plenciz, justified as
these have been by present knowledge. In the spirit of speculation
which was dominant in Europe and particularly in Germany in the latter
half of the eighteenth and the first half of the nineteenth centuries,
hypotheses did not stimulate research, but led to further
speculations. As late as 1820 Ozanam expressed himself as follows:
"Many authors have written concerning the animal nature of the
contagion of disease; many have assumed it to be developed from animal
substance, and that it is itself animal and possesses the property of
life. I shall not waste time in refuting these absurd hypotheses." The
theory of a living contagion was too simple, and not sufficiently
related to the problems of the universe to serve the medical
philosophers.
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