to them by successively larger and larger doses.  It is by this power, apparently, that the inoculation against hydrophobia produces its effect.  Material containing the hydrophobia poison (taken from the spinal cord of a rabbit dead with the disease) is injected into the individual after he has been bitten by a rabid animal.  The poisonous material in the first injection is very weak, but is followed later by a more powerful inoculation.  The result is that after a short time the individual has acquired the power of resisting the hydrophobia poisons.  Before the incubation period of the original infectious matter from the bite of the rabid animal has passed, the inoculated individual has so thoroughly acquired a tolerance of the poison that he successfully resists the attack of the infection.  This method of inoculation thus neutralizes the effects of the disease by anticipating them.

The method of treatment of hydrophobia met with extraordinarily violent opposition.  For several years it was regarded as a mistake.  But the constantly accumulating statistics from the Pasteur Institute have been so overwhelmingly on one side as to quiet opposition and bring about a general conviction that the method is a success.

The method of preventive inoculation has not been extensively applied to human diseases in addition to those mentioned.  In a few cases a similar method has been used to guard against diphtheria.  Among animals, experiment has shown that such methods can quite easily be obtained, and doubtless the same would be true of mankind if it was thought practical or feasible to apply them.  But, for reasons mentioned, this feature of preventive medicine will always remain rather unimportant, and will be confined to a few of the more violent diseases.

It may be well to raise the question as to why a single attack with recovery conveys immunity.  This question is really a part of the one already discussed as to the method by which the body cures disease.  We have seen that this is in part due to the development of chemical substances which either neutralize the poisons or act as germicide upon the bacteria, or both, and perhaps due in part to an active destruction of bacteria by cellular activity (phagocytosis).  There is little reason to doubt that it is the same set of activities which renders the animal immune.  The forces which drive off the invading bacteria in one case are still present to prevent a second attack of the same species of bacterium.  The length of time during which these forces are active and sufficient to cope with any new invaders determines the length of time during which the immunity lasts.  Until, therefore, we can answer with more exactness just how cure is brought about in case of disease, we shall be unable to explain the method of immunity.

Limits of preventive medicine.