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their environment. So long as they can find an environment in which the
constituents are ready made, they find the loss of ability to manufacture them
no disadvantage whatever. They are then able to reduce the size of their
bodies until finally they appear as the smallest of viruses, left only with the
function of reproduction. There is thus a sort of descending scale of sizes and
abilities amongst organisms.
The Battle of the Species.
Most of them are able to live and to reproduce themselves in the tissues
and on the mucus and skin surfaces of other living creatures. These other
creatures have developed mechanisms for defence against the attacks of the
parasitic organisms such as the production of antibodies against germ poisons,
special cells able to devour invading organisms, and mechanical devices such
as blinking, sneezing and coughing. But although we do not know what is the
exact nature of the intrinsic resistance of a cell to an organism, it is clear
it is only a relative resistance. The symptoms of an infectious disease can be
regarded as due to the damage done to cells when organisms feed on their
constituents and multiply within them. In poliomyelitis, for instance, the nerve
cell is disorganised whilst the virus is breeding inside it, with consequent
damage to the nerve fibre and resulting paralysis. Thus the virulence of an
organism is really the readiness with which it can obtain its food for growth
and multiplication from the cells of the host. As time goes on, the virulence
of an organism changes; it may increase, as in the influenza epidemic of 1918,
or it may decrease as scarlet fever has done this century. These variations in
virulence are due to mutations occuring at random in the offspring of microorganisms;
if the new strain has an increased virulence, the attack rate on
the hosts will be temporarily increased. It can never be permanently increased
to such an extent as to cause the deaths of all hosts, for then that species of
organism would perish. The test of fitness of an organism is thus long-term
survival. The ideal conditions for the survival and propagation of an organism
are that it should be readily transmissible from one person to another, that
almost everyone should become infected, and that whilst infected there should
not be any very great disability—indeed, preferably the patient should be able
to walk about. The common cold is a good example.
Changing Virulence.
The cause of the diminution of the virulence of pathogenic organisms varies
with different diseases. With tuberculosis, in the absence of treatment or
improved social conditions, the diminution may have to wait for genetic mutants
to arise in the human population who can withstand more ably the attacks;
this process may take five or six or more generations. Over the last hundred
years diphtheria has shown increases and decreases in severity with a periodicity
of about 30 years. The reason for the change is the ability of human beings to
develop immunity. Repeated small infections by the diphtheria bacillus can
produce an acquired immunity sufficient to withstand the evil effects of a large
and severe infection. We might ask why it is that if this mechanism is possible
in diphtheria, why is it not so in all diseases, and why are some of us liable to
be severely smitten.
Defensive Weapons.
The answer is that human beings have had to develop protective mechanisms
of a general kind and designed to deal with as many as possible of the
huge variety of harmful invaders always ready to attack. These mechanisms
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