244 INFECTION AND IMMUNITY 



alters in some way the resistance of the individual so that further 

 exposure to the infective agent is usually without menace, either for 

 a limited period after the attack, or for life. Resistance acquired in 

 this way is often spoken of as acquired immunity. 



The protection conferred by certain diseases against further attack 

 was recognized many centuries ago, and there are records which show 

 that attempts were made in ancient China and India to inoculate 

 healthy individuals with pus from small-pox pustules in the hope 

 of producing by this process a mild form of the disease and its 

 consequent immunity. 



Pasteur, before all others, thought philosophically about the 

 phenomena of acquired immunity, and, with adequate knowledge, 

 realized the possibility of artificially bestowing immunity without 

 inflicting the dangers of the fully potent infective agent. The first 

 observation which, made by him, purely accidentally, inspired the 

 hope of the achievement of such a result, occurred during his experi- 

 ments with chicken cholera. The failure of animals to die after inocula- 

 tion with an old culture of the bacilli of chicken cholera, fully potent 

 but a few weeks previously, pointed to the attenuation of these bacilli 

 by their prolonged cultivation without transplantation. With this 

 observation as a point of departure he carried out a series of investiga- 

 tions with the purpose of discovering a method of so weakening or 

 attenuating various incitants of disease that they could be introduced 

 into susceptible individuals without endangering life and yet without 

 losing their property of conferring protection. The brilliant results 

 achieved by Jenner, many years before, in protecting against smallpox 

 by inoculating with the entirely innocuous products of the pustules 

 of cowpox furnished an analogy which gave much encouraging support 

 to this prospect. 



The experimental work which Pasteur carried out to solve this 

 problem not only reaped a rich harvest of facts, but gave to science 

 the first and brilliant examples of the application of exact laboratory 

 methods to problems of immunity. 



ACTIVE IMMUNITY 



Active Artificial Immunity. The process of conferring protec- 

 tion by treatment with either an attenuated form or a sublethal quan- 

 tity of the infectious agent of a disease, or its products, is spoken of 



