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Page 45
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.
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