his body he can adapt it to the environment. The pathogenic bacterium
finds the living tissue hostile, its cells devour him, the tissue
fluids destroy him, and by means of the toxines he changes the
environment from that of living to dead tissue, or in other ways so
alters it that it is no longer hostile. The parasite has also means of
passive defence comparable to the armor of the warrior in the past. It
may form a protective mantle called a capsule around itself, which
serves to protect it from the action of the body fluids. Such capsule
formation is a very common thing in the pathogenic organisms, and they
are found only when these are growing in the body and do not appear in
cultures (Fig. 17-c).

It is evident that just as the parasite has his weapons of offence and
defence so has the host, otherwise there would be no recovery from
infectious diseases. Although many of the infectious diseases have a
high mortality, which in rare instances reaches one hundred per cent,
the majority do recover. In certain cases the recovery is attended by
immunity, the individual being protected to a greater or less degree
from a recurrence of the same disease. The immunity is never absolute;
it may last for a number of years only, and usually, if the disease be
again acquired, the second attack is milder than the primary. Probably
the most enduring immunity is in smallpox, although cases are known of
two and even three attacks; the immunity is high in scarlet fever,
measles, mumps and typhoid fever. The immunity from diphtheria is
short, and in pneumonia, although there must be a temporary immunity,
future susceptibility to the disease is probably increased. In certain
cases the immunity is only local; the focus of disease heals because
the tissue there has evolved means of protection from the parasite,
but if any other part of the body be infected, the disease pursues the
usual course. A boil, for example, is frequently followed by the