100 



a generation. In many cases, as in tuberculosis, anthrax, 

 and in hog cholera, the organisms have thus. been carried 

 through several hundred consecutive generations without 

 impairment of pathogenic properties. In other instances, 

 as in glanders, the organism does not find in our artificial 

 media the conditions favorable for its growth and as a 

 result it undergoes a physiological alteration so that the 

 cultures become less and less active till finally they cease 

 to have any effect on animals. This change in the physi- 

 ological properties of an organism known as attenuation 

 is frequently accompanied by a corresponding decrease 

 in the vitality of the growth so that, when the virulence is 

 wholly lost, the culture soon dies out. Sometimes, how- 

 ever, the organism adapts itself to the artificial media and 

 continues to grow although with diminished pathogenic 

 properties. 



The above four rules have been fully complied with 

 in a large number of infectious diseases. In others the 

 first two rules ;ire s;itij-fied bill the third is nor, owing to 

 the difficulty of obtaining a susceptible animal. Ay:ain, the 

 first rule maybe ihoonly one complied with, as in leprosy, 

 where the isolaiio'i of the organism has not, thus far, 

 been unquestionably successful. And again a large 

 number of infectious diseases remain, in which even the 

 presence of a specific organism has not been definitely 

 shown. 



Although many of the infectious diseases have been 

 shown to be due to bacteria, it must not be forgotten that 

 other low forms of plant and animal life possess sim- 

 ilar properties. Thus there are infectious diseases due to 

 fungi and also such as are due to animal parasites sporo- 

 zoa, etc. 



