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Page 46
There is variation in the susceptibility to infection in the different
races of man. If a race be confined to one habitat with close
intercourse between the people, such a race may acquire a high degree
of immunity to local diseases by a gradual weeding out of the
individuals who are most susceptible. A degree of comparative harmony
may be gradually established between host and parasite, as is the case
in wild animals. These have few diseases, the weak die, the resistant
breed; they harbor, it is true, large numbers of parasites, but there
is mutual adjustment between parasite and host. Diseases in animals
greatly increase under the artificial conditions of domestication.
Certain highly specialized breeds of cattle, as the Alderneys, are
much more susceptible to tuberculosis than the less specialized. The
high development of the variation which consists in a marked ability
to produce milk fat is probably combined with other qualities, shown
in diminished resistance to disease, and under natural conditions the
variation would not have persisted. The introduction of a new disease
into an isolated people has often been attended with dire
consequences. It is much the same thing with the introduction of
disease of plants. In Europe the brown-tail moth and the gypsy moth
produce continuously a certain amount of damage to the trees, but
their parasitic enemies have developed with them and check their
increase. These pests were brought to this country in which there were
no conditions retarding their increase and have produced great damage.
It is very difficult to estimate the degree of racial susceptibility.
The negro race seems to be more susceptible to certain diseases, such
as tuberculosis and smallpox, less so to others, as yellow fever,
malaria and uncinariasis. What are apparently differences in
susceptibility may be explained by racial customs. A statistical
inquiry into death in India from poisonous snakes might be interpreted
as showing a marked resistance on the part of the white to the action
of the venom, but it is merely a question of the boots of the whites
and the naked feet and legs of the natives. The relatively greater
frequency of smallpox in the blacks is due to the greater difficulties
in carrying out vaccination measures among them and the greater
opportunity for infection which results from their less hygienic life.
It has always been noted that when plague prevails in Oriental cities,
the natives are more frequently attacked than are Europeans. This does
not depend upon differences in susceptibility, but on the better
hygienic conditions of the whites which prevent the close relation to
rats and vermin by which infection is extended. There would be but
little extension of the hookworm disease in a community where shoes
were worn and the habits were cleanly.
It is by no means improbable that the formation of the habits of
civilization was influenced by infection. Most of these habits, such
as personal cleanliness, the avoidance of close contact, the demand
for individual utensils for eating and drinking, are all of distinct
advantage in opposing infection. Certain habits, on the other hand,
such as kissing, which probably represents the extension of a habit of
sexual origin, are disadvantageous and infection is often transmitted
in this way. In syphilitic infection the mouth forms one of the most
common localizations of the disease and may contain the causal
organisms in great numbers. This, the _spiroch�ta pallida_, is an
organism of great virulence, and man is the most susceptible animal.
The disease, like gonorrhoea, is essentially a sexual disease, the
primary location is in the sexual organs, and it is transmitted
chiefly by sexual contact. Of all the infectious diseases, it is the
one most frequently transmitted to the unborn child; in certain cases
the disease is transmitted, in others the developing foetus may be so
injured by the toxic products of the disease that various
imperfections of development result, as is shown in deformities, or in
conditions which render the entire organism or individual organs,
particularly the nervous system, more susceptible to injury. Following
the primary localization of the acquired form of the disease, there is
usually secondary localization in the mucous membrane of the mouth,
and the disease may be transmitted by kissing or by the use of
contaminated utensils. The habit of indiscriminate kissing is one
which might with great benefit be given up.
There is definite relation between age and the infectious diseases. In
general, susceptibility is increased in the young; young animals can
be successfully inoculated with diseases to which the adults of the
species are immune, and certain human diseases, such as scarlet fever,
measles and whooping cough, seem to be the prerogatives of the child.
It must be remembered, however, that one attack of these diseases
confers a strong and lasting immunity and children represent a raw
material unprotected by previous disease. Where measles has been
introduced into an island population for the first time, all ages seem
equally susceptible. All ages are equally susceptible to smallpox, and
yet in the general prevalence of the disease in the prevaccination
period it was almost confined to children, the adults being protected
by a previous attack. The habits and environment at different ages
have an influence on the opportunities for infection. There is
comparatively little opportunity for infection during the first year,
in which period the infant is nursed and has a narrow environment
within which infection is easily controlled. With increasing years the
opportunities for infection increase. When the child begins to move
and crawl on hands and knees the hands become contaminated, and the
habit of putting objects handled into the mouth makes infection by
this route possible. Food also becomes more varied, milk forms an
important part of the diet, and we are now appreciating the
possibilities of raw milk in conveying infection. With the enlarging
environment, with the school age bringing greater contact of the child
with others, there come greater opportunities for infection which are
partly offset by the increase in cleanliness. The dangers of infection
in the school period are now greatly lessened by medical inspection
and care of the school children. In the small epidemic of smallpox
which prevailed in Boston from 1881 to 1883, there was a sharp decline
in the incidence of the disease in children as soon as the school age
was reached, this being due to the demand of vaccination as a
condition for entrance into the schools. Many of the infectious
diseases are much milder in children than in adults. This is the case
in typhoid fever, malaria and yellow fever. The comparative immunity
of the natives to yellow fever in regions where this prevails seems to
be due to their having acquired the disease in infancy in so mild a
form that it was not recognized as such.
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