232 



BIOLOGY OF SPECIALIZED GROUPS 



All the above-mentioned facts and experiments serve to 

 demonstrate the conclusion which can be definitely stated ; 

 namely, that the limits of the " pathogenic bacteria" are not 

 definite. 



Diseases. Infectious and Contagious. All diseases pro- 



FIG. 49. Leishman-Donovan bodies of kala-azar. It is supposed that at cer- 

 tain stages these bodies are flagellated similar to Figure 48. Williams. 



duced by bacteria and other microorganisms are called in- 

 fectious diseases. Many definitions have been suggested by 

 pathologists for the term " infection." According to Ziegler, 

 infection is " the entrance of bacteria into the body and their 

 increase there." As McFarland says, this usually means an 

 entrance into the tissues of the body and a multiplication 

 of the bacterial cells in the tissues. Some pathogenic bac- 



