162 ESSAYS ON BACTERIOLOGY. 



and white mice against glanders. Dosage and other 

 conditions being the same, white rats yield to infec- 

 tion with anthrax most readily, gray rats next, and 

 black rats least. Many more examples might be 

 given illustrating the principle that the growth of dis- 

 ease germs may be more or less influenced by the race 

 or species of animal infected. 



Second, just as race seems to play a part in the in- 

 fectious diseases, so does age. For instance, very 

 young infants seem to possess a relative insuscepti- 

 bility to measles and scarlatina; later their vulnera- 

 bility increases, and again seems to decline with ad- 

 vancing years. 



The same thing may be demonstrated in the labor- 

 atory. It is a general rule, well known to bacterio- 

 logical experimenters, that young animals are more 

 readily infected with cultures of bacteria than are the 

 old. Certain animals, which at a mature age show a 

 strong degree of immunity against a disease, may be 

 comparatively easily inoculated in early life. In this 

 respect also, therefore, experiments with bacteria con- 

 firm the observations at the bedside. 



Third. Into the production of individual suscep- 

 tibility many and various factors enter, some of which 

 we understand, while others are still mysteries. Some 

 are general, others local in action. Thus it has long 

 been observed that persons suffering from diabetes 

 mellitus are specially prone to the development of sup- 

 puration and tuberculosis; that erysipelas commonly 



