2 BIOLOGICAL LECTURES. 



of these belong to the vegetable, others to the animal king- 

 dom. They are found in the former, among the fungi and 

 bacteria, and in the latter, among the protozoa, vermes, and 

 arthropods. While, however, the term pathogenic micro-organ- 

 isms is arbitrarily applied to all the vegetable parasites, among 

 the animal parasites only the protozoa should properly come 

 under this category. 



Infectious diseases, then, are such as are caused by pathogenic 

 micro-organisms, i.e. by fungi, bacteria, and protozoa. In 

 speaking of the bacterial origin of diseases many writers apply 

 the term bacteria to micro-organisms of animal as well as of 

 vegetable origin ; but it must be remembered that protozoa are 

 not bacteria, although the term pathogenic micro-organisms 

 can be properly applied to both. I wish, therefore, to empha- 

 size the fact, that besides the diseases of bacterial origin, there 

 are also others which are caused by organisms belonging to 

 the animal kingdom. Typical examples of this class are found 

 in the different forms of malarial fever which are caused by 

 the invasion of the blood and organs by organisms belonging 

 to the group of protozoa. 



It must, however, be admitted that the diseases caused by 

 vegetable parasites are best understood. This fact is easily 

 explained by our present successful methods employed for the 

 propagation of these parasites outside the body, whereas as yet 

 the pathogenic protozoa have not been obtained in the form of 

 pure cultures outside the bodies of infected animals. The 

 growth and multiplication of the pathogenic micro-organisms 

 are associated in many instances with the production of cer- 

 tain substances of a toxic nature, these poisons, or toxins as 

 they are called, playing a great role in the causation of disease. 

 Certain non-pathogenic or saprophytic micro-organisms in the 

 course of their growth are also capable of producing poisonous 

 products. There is, however, this marked difference between 

 these two classes of agents, namely, that whereas the former 

 are capable of living and of manufacturing the toxins within 

 the living body, the latter can subsist only in the presence 

 of dead material. Examples of poisoning by the products 

 developed by saprophytic bacteria are found in the accounts, 



