DISEASE CARRIERS.--THE RELATION BETWEEN SPORADIC CASES OF INFECTIOUS
DISEASE AND EPIDEMICS.--SMALLPOX.--CEREBRO-SPINAL
MENINGITIS.--POLIOMYELITIS.--VARIATION IN THE SUSCEPTIBILITY OF
INDIVIDUALS.--CONDITIONS WHICH MAY INFLUENCE SUSCEPTIBILITY.--RACIAL
SUSCEPTIBILITY.--INFLUENCE OF AGE AND SEX.--OCCUPATION AND
ENVIRONMENT.--THE AGE PERIOD OF INFECTIOUS DISEASES.
We have seen that insects serve as carriers of disease in two ways: in
one, by becoming contaminated with organisms they serve as passive
carriers, and in the other they undergo infection and form a link in
the disease. The more recent investigations of modes of transmission
of infectious diseases have shown that man, in addition to serving
while sick as a source of infection, may serve as a passive carrier in
two ways. For infection to take place not only must the pathogenic
organism be present, but it must be able to overcome the passive and
active defences of the body and produce injury. Pathogenic organisms
may find conditions favorable for growth on the surfaces of the body,
and may live there, but be unable to produce infection, and the
individual who simply harbors the organisms can transmit them to
others. Such an individual may be a greater source of infection than
one with the disease, because there is no suspicion of danger. The
organisms which thus grow on the surfaces have in some cases been
shown to be of diminished virulence, but in others have full
pathogenic power. Such passive carriers of infection have been found
for a number of diseases, as cerebro-spinal meningitis, diphtheria,
poliomyelitis and cholera. In all these cases the organisms are most
frequently found in those individuals who have been exposed to
infection as members of a family in which there have been cases of
disease. The other sort of carrier has had and overcome the disease,
but mutual relations have been established with the organism which
continues to live in the body cavity. Diphtheria bacilli usually
linger in the throat after convalescence is established, and until
they have disappeared the individual is more dangerous than one
actually sick with the disease. Health officers have recognized this
in continuing the quarantine against the disease until the organism
disappears. In typhoid fever bacilli may remain in the body for a long
time and be continually discharged, as in the well-known case of
"typhoid Mary."[1]
Single cases of certain infectious diseases may appear in a community
year after year, and at intervals the cases become so numerous that
the disease is said to be epidemic. Such a disease is smallpox. This
is a highly infectious disease, towards which all mankind is
susceptible. Complete protection against the disease can be conferred
by Jenner's discovery of vaccination. The disease becomes modified
when transferred to cattle, producing what is known as cowpox, in
which vesicles similar to those of smallpox appear on the skin. The
inoculation of man with the contents of such a vesicle produces a mild
form of disease known as vaccinia, which protects the individual from
smallpox. This protection is fully as adequate as that produced by an
attack of smallpox, and we are warranted in saying that if thorough
vaccination, or the inoculation with vaccinia, were carried out
smallpox would disappear. There are great difficulties in the way of
carrying out effective vaccination of the whole population, which are
accentuated by the active opposition of people who are ignorant and
wilfully remain so. There exists in every state a number of people
unprotected by vaccination, and among these single cases of smallpox
appear. The unprotected individuals gradually increase in number,
forming an inflammable material awaiting the spark or infection which
produces a conflagration in the one case and an epidemic in the other.
Cerebro-spinal meningitis is another example of a disease which exists
in sporadic and epidemic form. This disease is caused by a small
micrococcus, the organisms joined in pairs. The seat of the disease is
in the meninges or membranes around the brain and spinal cord. The
micrococci enter the body from the throat and nose, and either pass
directly from here into the meninges, or they enter into the blood and
are carried by this into the meninges. The organisms are easily
destroyed and cannot long survive the conditions outside the body, so
that for infection to take place the transmission must be very direct.
Carriers who have the organisms in the throat, but who do not have the
disease, are the principal agents in dissemination. The mortality is
high, and even in recovery permanent damage is often done to the brain
or to the organs of special sense. Sporadic cases constantly occur in
small numbers, and it is difficult or impossible to trace any
connection between these cases. At varying intervals, often twenty
years intervening, an epidemic appears which sometimes remains local
in a city or state, sometimes extends to adjoining cities or states,
and may even extend over a very large area. In the epidemics the
mortality is much higher than in the sporadic cases. The same
explanation given for smallpox cannot apply here, for there is not a
similar accumulation of susceptible material. We know there is a great
deal of variation in the virulence of the different pathogenic
organisms, and the virulence can be artificially increased and
diminished. In epidemics of meningitis the virulence of the organisms
is increased, as is shown by the greater mortality. It is highly
probable that such epidemics are due to changes which arise in the
organisms from causes we do not know and which increase their capacity
for harm. It is possible that such a change would convert a carrier
into a case of disease, the organism acquiring greater powers of
invasion. Such a strain of organisms arising in one place and
producing an epidemic could be transported to another locality and
exert the same action, or similar changes in the organisms could arise
simultaneously in a number of places. Analogies to such conditions are
given in plants. In certain plants it has been shown that from unknown
causes there appears a tendency to the production of variations. A
very beautiful herbaceous peony known as "Bridesmaid" after having
grown for a number of years in single form, in one year wherever grown
suddenly became double. The peculiar thing with the lower unicellular
organisms is that the changes which so arise do not tend to become
permanent, the organism reverts to its usual character, the disease to
its sporadic type.
A very fatal form of poliomyelitis has for a number of years prevailed
in Sweden. In the United States there have been continually a number
of single cases of the disease, and it is not impossible that a more
pathogenic strain of the organism has developed in Sweden and has been
imported into this country, giving rise to the much greater extension
of the disease in a number of places.
The most cursory study of the infectious diseases shows that there is
great variation in the susceptibility of individuals. Even in the most
severe epidemics all are not equally affected, some escape the
infection, others have the disease lightly, others severely, some die.
Chance enters into this, but plays a small part, for the same varying
individual susceptibility is shown experimentally. If a given number
of animals of the same species, age and weight, even those from the
same litter, be inoculated with a given number of bacteria shown to be
pathogenic for that species, the results differ. If the dose be
necessarily fatal, death will take place at intervals; if a dose
smaller than the fatal be used, some animals will die, others will
recover. The defences of the organism being centred in the activity of
the living tissue, any condition which depresses cell activity may
have an effect in increasing susceptibility to infection. Animals
which ordinarily are not susceptible to infection with a certain
organism may be made so by prolonged hunger, or fatigue, by the
influence of narcotics, by reduction of the body temperature, by loss
of blood. In man prolonged fatigue, cold, the use of alcohol to excess
and even psychic depression increases susceptibility. It has been
shown that such conditions are accompanied by a diminution in the
power of the blood to destroy bacteria.
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 _spirochæta 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.
The infectious diseases are preëminently the diseases of the first
third of life. After the age of forty man represents a select
material. He has acquired immunity to many infections by having
experienced them. Habits of life have become fixed and there is a
general adjustment to environment. The only infectious disease which
shows no abatement in its incidence is pneumonia, and the mortality in
this increases with age. Between thirty-five and fifty-five man stands
on a tolerably firm foundation regarding health; after this the age
atrophies begin, the effects of previous damage begin to be apparent,
and the tumor incidence increases.
FOOTNOTE:
[1] This was the case of a woman, by occupation a cook, whose numerous
exchanges of service were accompanied by the appearance of cases of
typhoid fever in the families. This became so marked that an
examination was made and she was found to be a typhoid carrier and as
such constantly discharging typhoid bacilli. She is now isolated.