594 INFECTION AND IMMUNITY. 



ing little or no tissue change, although they retain 

 their virulence for other animals (guinea-pigs). 

 No animal exceeds the guinea-pig in its susceptibil- 

 ity to this disease. Goats and sheep are fairly re- 

 sistant, and the same is probably true of the horse, 

 although its artificial infection is not difficult. 

 That different varieties of a species may vary in 

 their susceptibility is illustrated by the field mouse, 

 Racial and which is highly susceptible, and the white mouse. 



Individual , . , . i i i A 1.1 -. -i 



variations, which is relatively immune. Although similar 

 variations may exist among different races of men, 

 they are not readily demonstrated. The high sus- 

 ceptibility which appears to exist among certain 

 races, as the negro, may be explained in part by un- 

 hygienic methods of living, in which safeguards 

 against infection are not taken. 



The discovery of healed or healing tuberculous 

 foci in 70 to 90 per cent, of all autopsies, in con- 

 trast to the 15 to 20 per cent, of deaths from tuber- 

 culosis, shows that susceptibility and immunity are 

 subject to marked individual variations. The 

 ability of an individual to overcome a tuberculous 

 infection is referred in a vague way to an unusual 

 resistance on his part; his defensive powers are 

 said to be strong. Although we remain to a large 

 extent in the dark concerning these defensive 

 powers, they seem to rest chiefly in the ability of 

 the tissues to destroy the bacilli; that is, the re- 

 sistance is antibacterial. Many bacilli may be de- 

 stroyed by leucocytes or endothelial cells before 

 they are able to cause tissue changes. It was stated 

 previously that healing in many instances depends 

 on isolation of the focus by epithelioid, lymphoid 

 and plasma cells, and by connective tissue. On 

 general grounds we may assume that a tissue reac- 



