424 Major P. S. Leiean [April 23, 



jeopardized the lives of the latter by denouncing a method which 

 offends their prejudices. 



To the credit of the men it may be said that the great majority 

 of them were sufficiently intelligent to be uninfluenced by these 

 attempts, but the small minority who were influenced exposed them- 

 selves and the forces to which they belonged to so great dangers of 

 acquiring enteric fever, that it was no longer possible to ignore the 

 efforts of the busybodies. It became necessary that at least the 

 civilian practitioners who volunteered their services should be fully 

 informed as to the rationale of this method, and be fully armed with 

 precise data supporting its claims to success in protecting the forces 

 from the most dangerous scourge of armies in the field. 



It is further advisable that a frank statement of the true merits 

 of the case in favour of universal application in the army of this 

 means of protection should be given a publicity which will enlist the 

 support of a weight of public opinion fatal to the hopes of the faddists. 



The problems relating to anti-typhoid inoculation can only be 

 approached by an initial consideration of certain facts established by 

 research into the phenomena of immunity — a research which has 

 proved one of the most brilliantly successful laboratory studies. 

 Unlike so many barren triumphs of the laboratory, this research has 

 led to important practical results, for from it has been evolved the 

 most effective contribution of preventive medicine to our means of 

 safeguarding the lives of our men in camp and field. 



I propose, therefore, to consider to-night : — 



1. The general principles of active immunization. 



2. The application of those principles to the problem of protec- 

 tion against enteric (typhoid) fever. 



3. The scientific and practical results obtained hj prophylactic 

 inoculation. 



A. — The General Principles of Active Immunization. 



Beginning, then, with the general principles of immunization, 

 one defines immunity as relative insusceptibility to disease, and in 

 doing so stress is laid on the qualifying adjective " relative," for 

 immunity is seldom, if ever, absolute. 



Immunity, as thus defined, varies according to — 



(rt) The number of invading organisms. It is a matter of 

 common knowledge that, thanks to dilution of the dose of infec- 

 tive material by good ventilation, relatively few among the many 

 hundreds of thousands of persons who inhale tubercular bacilli 

 acquire tubercular disease. 



{h) The route of invasion. It is interesting to note that, while 

 cholera vibriones are intensely virulent in the intestine and innocuous 



