EELATION TO INFECTIOUS DISEASES. 97 



complied with, the germ is a cause of the disease, .but our 

 range of observation must be much wider than it now is 

 before we can say that the given germ is the only cause of 

 the disease. 



We believe that those few infectious diseases, such as 

 anthrax and tuberculosis, which have such well-marked, 

 typical, clinical histories, are due to equally well-marked 

 and morphologically distinct microorganisms which can be 

 recognized by microscopical study alone ; but we do not 

 believe that this is true in diseases showing such wide vari- 

 ations in symptoms as is the case in typhoid fever and 

 cholera infantum. 



In all cases, we insist that the true test of the specific 

 character of a germ is to be made with its chemical pro- 

 ducts. A given bacterium may not multiply in the circu- 

 lating blood of a dog, and failure to do so is by no means 

 proof that the same organism might not cause disease in 

 man ; but every germ which causes disease in man does so 

 by virtue of its chemical products, and if these be isolated / 

 and injected into the dog in sufficient quantity a poisonous/ 

 effect will be produced. In the study of the bacteriology; 

 of the infectious diseases, the third and fourth of Koch's 

 rules have not been complied with in many diseases on 

 account of the insusceptibility of the lower animals. The 

 majority of investigators, meeting with this difficulty, have 

 been inclined to rest content with the first two rules, and 

 to conclude that when a given germ is constantly present 

 in a given disease, and not found in other diseases, that it 

 is the cause of the disease with which it is associated. In- 

 deed, we find so good an authority as Welch stating that 

 the successful inoculation of animals is not necessary in 

 order to prove the causal relationship of a germ to a disease. 

 In 1889, Vaughan suggested that in those instances in 

 which the third and fourth of Koch's rules cannot be 

 complied with on account of the insuscx'ptibility of the 

 lower animals, it must be shown that the germ can pro- 

 duce chemical poisons which will induce in the lower ani- 

 mals in an acute form the characteristic symptoms of the 



