8 INFECTION AND IMMUNITY. 



does the susceptibility of one species depend on 

 the absence of those properties which characterize 

 the natural immunity of another species? Pas- 

 teur had observed that if he grew the microbe of 

 chicken cholera in a liquid medium for some time, 

 then removed the bacteria by filtration, the fluid 

 became unfit for the further growth of the organ- 

 ism on subsequent reinoculation. That is, the 

 nutrient material had been used up; and he sug- 

 gested that this is the case in the body of an ani- 

 mal. Having undergone the infection, suitable 

 nutrient material for the microbe is used up, and 

 recovery ensues. The prolonged absence of the 

 proper nutritious substances would account for 

 the more or less permanent nature of the acquired 

 immunity. This conception, the exhaustion 

 theory, at one time shared by Koch and Klebs, is 

 still represented in an altered form by Baumgar- 

 ten, who speaks of an unfavorable culture medium 

 as representing the condition of the immune body, 

 which, of course, is broadly true. 



Chauveau was the author of another historic 

 theory of acquired immunity (the noxious reten- 

 tion theory), which maintained that during the 

 course of a disease the bacteria produce substances 

 in the presence of which they can not develop 

 further; consequently recovery takes place, and 

 the continued presence of these noxious substances 

 renders another attack of the disease impossible. 

 Although it is true that bacteria do not grow well 

 in their own metabolic products, theories of im- 

 munity on this and similar bases are not in ac- 

 cord with the fact that immunity may be of great 

 duration, and that it may be conferred by the 



