CAUSES OF DISEASE. 7 



becoming evident that both protozoa and moulds are 

 of almost, if not of equal importance, as causative 

 factors in infectious diseases; and that besides these, 

 there are other microscopic and ultra-microscopic 

 forms of life, not included in the three groups, that are 

 playing a similar role. For these reasons we conclude 

 that the only defensible definition of an infectious agent 

 is that it is either an animal or vegetable organism, 

 microscopic in size, which produces an infectious 

 disease. It will be observed, from the point of view 

 of this definition, that we classify the organisms that 

 produce infectious diseases not by their place in the 

 animal or vegetable kingdom, but by the effects they 

 produce in the living body. 



The animate agents, it will be remembered, we 

 divided into two classes, parasites, and infectious agents. 

 The designations have reference both to the manner 

 in which the agents live upon the body, and the phenom- 

 ena their presence give rise to; their place in either 

 animal or vegetable kingdom is again disregarded. 

 The mode of action of the infectious agent is character- 

 istic, and markedly different from that of the parasite. 

 When it enters a living body, it aims directly at the 

 destruction of the latter. It multiplies rapidly, tends 

 to scatter its broods throughout the tissues, and all 

 the while gives off the most powerful poisons known. 

 This agent is wickedly implacable, neither giving nor 

 asking quarter. ThQ battle that it wages with the body 



