78 ESSAYS ON BACTERIOLOGY. 



the specific bacteria, but perhaps little of importance 

 in addition to what has already been said. I turn, 

 therefore, to a brief survey of that rising field of work 

 in which all are deeply interested, namely, the inves- 

 tigations upon immunity from and protection against 

 the infectious diseases. 



An immense amount of work is being done in this 

 direction; hap-hazard experiments are becoming 

 fewer, and the whole matter is steadily being reduced 

 to a more scientific basis. We now recognize several 

 ways of securing a more or less lasting immunity 

 against many of these diseases, and there is opening up 

 a new field, already promising, in the way of cure for 

 those laboring under an attack. It would require a 

 long time to even briefly review this great mass of 

 work, and present its results. I can, therefore, only 

 note some interesting and leading points. As I have 

 said, there are several ways by which a more or less 

 lasting immunity may be secured. 



Eirst. By inoculation of such amounts of living 

 and virulent cultiires of the microbe causing the dis- 

 ease as shall fall short of a fatal result. An example 

 of this was the practice of inoculation against small- 

 pox, which preceded the far safer and about as equally 

 successful vaccination worked out by Jenner. 



Second. Inoculation of the specific microbe partly 

 or wholly attenuated in virulence, this attenuation be- 

 ing secured by various means, such as culture under 

 peculiar conditions, or passing the germs through the 



