116 ESSAYS ON BACTERIOLOGY. 



of these theories have some measure of truth, but all 

 have failed, until recently, to explain the facts. The 

 problem has now been worked out to a fairly satis- 

 factory conclusion in the establishment of the toxin 

 theory. We now know that most of the results "of 

 bacterial action are explained by the production in 

 the course of their growth of certain new substances. 

 If these substances are injurious or poisonous to the 

 living body, the bacteria producing them are said to 

 be toxic; if they are produced during the growth of 

 the microbes within the living body, such microbes 

 are said to be infectious. If the toxin-forming organ- 

 isms are reg-ularly associated as causative agents with 

 a peculiar or specific disease, they are called specific 

 bacteria. Thus the definition and classification of 

 many diseases has become more accurate, more ra- 

 tional, because built more nearly upon the basis of 

 causation. Infectious diseases are those which are 

 caused by the presence and growth, within the body, 

 of living producers of poisons. Specific infectious 

 diseases are those each of which is caused by a par- 

 ticular living toxin-producer, a particular or specific 

 bacterium. For example, the condition commonly 

 known as blood-poisoning may be due to any of a 

 number of bacteria, while the peculiar and specific dis- 

 ease diphtheria, with its peculiar poisoning of the 

 blood, is believed to be caused by the specific diph- 

 theria bacillus alone. An important corollary to such 

 a doctrine is found in this: that if, for example, the 



