TEXT-BOOK OF BACTERIOLOGY. 139 



escaped notice that manj^ of the deductions already laid down 

 stand on a weak foundation; that a firm footing exists only here 

 and there, our knowledge resembling a patchwork and being 

 greatly in want of additions and completions. 



II. THEORIES OF IMMUNITY. 



Further information in the matter just treated of, especially the 

 question of the peculiar action of the pathogenic bacteria, is very 

 desirable. It is immediately connected with another matter, darker 

 perhaps and more mysterious, but certainly far more important 

 for us, because it touches the great problem of the cure of infec- 

 tious diseases. Certain powers of resistance which the body em- 

 ploys against the parasites must be conquered by the latter if they 

 are to multiply and make use of their powers. We have also seen 

 that we can influence the relations of the two hostile powers and 

 turn the scale in favor of the one or the other : in favor of the 

 parasites if we diminish the body's power of resistance, for exam- 

 ple, by introducing injurious substances such as sublimate, pyro- 

 gallic acid, the excretions of other micro-organisms, or, as in the ex- 

 periments of Leo and Behring, by a change in the entire process of 

 assimilation; in favor of the body if we alter the nature of the bac- 

 teria and weaken their special products. But two further possi- 

 bilities are here present. The scale will turn in favor of the micro- 

 organisms if we succeed in sufficiently strengthening the bacteria, 

 an experiment which, as already mentioned, has not yet been 

 successful. On the contrary, the body gets an advantage if we can 

 succeed in increasing its natural power of resistance, in heightening 

 its bacteria-killing force, and in this particular field recent investi- 

 gations have engaged with remarkable energy and, as we may 

 perhaps say, with remarkable success. 



If particular animals are by nature safe from infection by par- 

 ticular micro-organisms, the reason is that they possess a. high 

 power of resistance in the constitution of their tissue-juices or in 

 the behavior of their cells. Besides this kind of non-susceptibility, 

 this inborn immunity, there is also an acquired immunity. We 

 know a whole series of diseases, particularly the exanthematic 

 affections, such as small-pox, measles, scarlet fever, etc., which 

 usually attack the organism but once and fortify it against after- 

 attacks — i.e., they place it in a like position with that of individuals 

 previously rendered non-inoculable. 



The immunity thus brought about by nature may also be pro- 

 duced artificially. There are proofs, for instance, that in China 

 more than 3,000 years ago men endeavored to combat that terrible 



