These facts are certainly surprising.  If one had been asked to suggest the least likely theory to explain recovery from disease, he could hardly have found one more unlikely than that the body cells developed during the disease an antidote to the poison which the disease bacteria were producing.  Nevertheless, it is beyond question that such antidotes are formed during the course of the germ diseases.  It has not yet been shown in all diseases, and it would be entirely too much to claim that this is the method of recovery in all cases.  We may say, however, in regard to bacterial diseases in general, that after the bacteria enter the body at some weak point they have first a battle to fight with the resisting powers of the body, which appear to be partly biological and partly chemical.  These resisting powers are in many cases entirely sufficient to prevent the bacteria from obtaining a foothold.  If the invading host overcome the resisting powers, then they begin to multiply rapidly, and take possession of the body or some part of it.  They continue to grow until either the individual dies or something occurs to check their growth.  After the individual develops the renewed powers of checking their growth, recovery takes place, and the individual is then, because of these renewed powers of resistance, immune from a second attack of the disease for a variable length of time.

This, in the merest outline, represents the relation of bacterial parasites to the human body But while this is a fair general expression of the matter, it must be recognised that different diseases differ much in their relations, and no general outline will apply to all They differ in their method of attack and in the point of attack.  Not only do they produce different kinds of poisons giving rise to different symptoms of poisoning; not only do they produce different results in different animals; not only do the different pathogenic species differ much in their power to develop serious disease, but the different species are very particular as to what species of animal they attack.  Some of them can live as parasites in man alone; some can live as parasites upon man and the mouse and a few other animals; some can live in various animals but not in man; some appear to be able to live in the field mouse, but not in the common mouse; some live in the horse; some in birds, but not in warm-blooded mammals; while others, again, can live almost equally well in the tissues of a long list of animals.  Those which can live as parasites upon man are, of course, especially related to human disease, and are of particular interest to the physician, while those which live in animals are in a similar way of interest to veterinarians.

Thus we see that parasitic bacteria show the widest variations.  They differ in point of attack, in method of attack, and in the part of the body which they seize upon as a nucleus for growth.  They differ in violence and in the character of the poisons they produce, as well as in their power of overcoming the resisting powers of the body.  They differ at different times in their powers of producing disease.  In short, they show such a large number of different methods of action that no general statements can be made which will apply universally, and no one method of guarding against them or in driving them off can be hoped to apply to any extended list of diseases.