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 based largely
on conjecture, the one brilliant exception being the discovery of
vaccination by Jenner in 1798.

The host possesses the passive defences of the surfaces which have
already been considered. The first theories advanced in explanation of
immunity were influenced by what was known of fermentation. One, the
exhaustion theory, assumed that in the course of disease substances
contained in the body and necessary for the growth of the bacteria
became exhausted and the bacteria died in consequence. Another, the
theory of addition, assumed that in the course of the disease
substances inimical to the bacteria were formed. Both these theories