INFECTIOUS DISEASES. 627 



ganism in the body of a susceptible animal accumulate during the 

 attack and are subsequently retained, and being prejudicial to the 

 growth of the particular micro-organism which produced them, a 

 second infection can not occur. Support for this theory has been 

 found by its advocates in the fact that various processes of fer- 

 mentation are arrested after a time by the formation of sub- 

 stances which restrain the development of the micro-organisms to 

 which they are due. But in the case of a living animal the con- 

 ditions are very different, and it is hard to conceive that adventi- 

 tious products of this kind could be retained for years, when in 

 the normal processes of nutrition and excretion the tissues and 

 fluids of the body are constantly undergoing change. Certainly 

 the substances which arrest ordinary processes of fermentation 

 by their accumulation in the fermenting liquid, such as alcohol, 

 lactic acid, phenol, etc., would not be so retained. But we can 

 not speak so positively with reference to the toxic albuminous 

 substances which recent researches have demonstrated to be pres- 

 ent in cultures of some of the best-known pathogenic bacteria. 

 It is difficult, however, to believe that an individual who has 

 passed through attacks of half a dozen different infectious dis- 

 eases, carries about with him a store of as many different chemi- 

 cal substances produced during these attacks, and sufficient in 

 quantity to prevent the development of the several germs of 

 these diseases. Nor does the experimental evidence relating to 

 the action of germicidal and germ-restraining agents justify the 

 view that a substance capable of preventing the development 

 of one micro-organism should be without effect upon others of 

 the same class ; but if we accept the retention hypothesis, we 

 must admit that the inhibiting substance produced by each par- 

 ticular pathogenic germ is effective only in restraining the devel- 

 opment of the microbe which produced it in the first instance. 



Moreover, if we suppose that the toxic substances which give 

 pathogenic power to a particular micro-organism are retained in 

 the body of an immune animal, we must admit that the animal 

 has acquired a tolerance to the pathogenic action of these toxic 

 substances, for their presence no longer gives rise to any morbid 

 phenomena. And this being the case, we are not restricted to 

 the explanation that immunity depends upon a restraining in- 

 fluence exercised upon the microbe when subsequently intro- 

 duced. 



Another explanation offers itself, viz., that immunity depends 

 upon an acquired tolerance to the toxic products of pathogenic bac- 

 teria. This is a view which the writer has advocated in various 

 published papers since 1881. In a paper contributed to the 

 American Journal of the Medical Sciences in April, 1881, it is pre- 

 sented in the following language : " The view that I am endeavor- 



