ot 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
appearance of similar boils in the vicinity due to the infection of
the skin by the micrococci from the first boil, which by dressings,
etc., have become spread over the surface.

The natural methods of defence of the host against the parasites have
formed the main subject in the study of the infectious diseases for
the last twenty years. Speculation in this territory has been rife and
most of it fruitless, but by patient study of disease in man and by
animal experimentation there has been gradually evolved a sum of
knowledge which has been applied in many cases to the treatment of
infectious diseases with immense benefit. Research was naturally
turned to this subject, for it was evident that the processes by which
the protection of the body was brought about must be known before
there could be a really rational method of treatment directed towards
the artificial induction of such processes, or hastening and
strengthening those which were taking place. Previous to knowledge of
the bacteria, their mode of life, their methods of infection and
knowledge of the defences of the body, most of the methods of
prevention and treatment of the infectious diseases was bas