them are being reaped. We have learnt that except in the case where some vital organ has been destroyed or fatally injured so that it can no longer perform its functions, the struggle between life and death in the vast proportion of cases is a fight with our omnipresent enemies the microbes. Thanks to the diligent pursuit of knowledge by experiment there has grown up since his time a wealth of knowledge of the nature of these enemies and the means by which Nature defends us from their attacks. In some cases the body responds to the presence of the microbes by making changes in the blood which lead to their extinction and these changes remain after the attack which gave rise to them has passed away and thus the body is left more or less permanently secure from further attacks or to use the consecrated phrase "becomes immune." In other cases where the microbe kills by generating a definite chemical poison, the body generates an equally definite chemical antidote which neutralizes it and keeps the body unharmed until the invader has lost its
