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.