92 BACTERIAL POISONS. 



what appears to us, in the present state of our knowledge, 

 a correct definition of an infectious disease. 



An infectious disease arises when a specific, pathogenic 

 microorganism, having gained admittance to the body, and 

 having found the conditions favorable, grows and multi- 

 plies, and in so doing elaborates a chemical poison which 

 induces its characteristic effects. 



In the systemic infectious diseases, such as anthrax, 

 typhoid fever, and cholera, this poison is undoubtedly taken 

 into the general circulation, and affects the central nervous 

 system. In the local infectious diseases, such as gonorrhoea, 

 and infectious ophthalmia, the principal action of the poison 

 seems to be confined to the place of its formation. Though 

 even in these, when of a specially virulent type, the effects 

 may extend to the general health. It may be that in some 

 diseases the chemical poison has both a local and a systemic 

 effect. Thus, it is by no means certain that the ulceration 

 of typhoid fever is due directly to the bacillus. On the 

 other hand, it is altogether probable that the anatomical 

 changes in the intestine result from the irritating effects of 

 the poison at the place of its formation. 



With the proof, that the deleterious effects wrought by 

 germs are due to chemical poisons elaborated l)y them 

 during their growth, admitted, let us inquire what proper- 

 ties a microorganism must possess before it can be said to 

 be the specific cause of a disease. The four rules of Koch 

 have been generally conceded to be sufficient to show that 

 a given germ is the sole and sufficient cause of the disease 

 with which that germ is associated. Briefly, these rules are 

 as follows : 



1 . The germ must be present in all cases of that disease. 



2. The germ must be isolated from other organisms 

 and from all other matter found with it in the diseased 

 animal. 



3. The germ tiius freed from all foreign matter must, 

 when properly introduced, pr<xluce the disease in healthy 

 animals. 



4. The microorganism must be found properly dis- 



