INTRO.] MICRO-ORGANISMS AND DISEASE 5 



establish the correctness of the theory of contagivin vivum. 

 Many facts have since come to Hght which necessitate a 

 relaxation of these rigid rules. As then so also now these 

 principles hold good, if the true causative relation between 

 a particular microbe and a particular disease is to claim 

 unequivocal acceptance, but there are a variety of conditions 

 under which such rigorous proof is impossible. Koch 

 himself has seen this in the case of cholera Asiatica. As 

 he himself so also others before and after him have 

 accepted as correct the everyday experience that Asiatic 

 cholera is a disease of the human subject only, that 

 domestic animals under natural conditions, in localities 

 where cholera is endemic or in localities where epidemics 

 prevail, have never been known to have been subject to 

 this disease. It is therefore obvious that since no animal 

 can be said to be susceptible to cholera in the sense in 

 which man is, the third and fourth points above stated, in 

 furnishing proof positive, cannot be fulfilled. True, under 

 certain modes of experimentation with cultures of the 

 cholera vibrio (see the Chapter on Cholera), guinea-pigs are 

 capable of developing an acute fatal disease ; but under the 

 same methods of experimentation other microbes, not 

 connected with cholera or any other disease, produce the 

 identical results or results differing only in degree. 



Again, take the case of typhoid fever : the microbe 

 which is found in the tissue of the spleen and mesenteric 

 glands in large numbers in cases of typhoid fever only is, 

 from its constant occurrence and its special biological 

 characters, justly considered to be the microbe of that 

 disease, but since animals are not susceptible to typhoid 

 fever, the two conditions mentioned sub 3 and 4 cannot 

 be verified. The pathogenic action which this microbe 

 is capable of exerting on guinea-pigs when injected sub- 



