IMMUNITY. 177 



sheep, as a rule, are easily infected with anthrax, this disease 

 seldom occurs in sheep of the Algerian variety or race. The 

 immunity which belongs to a race, but not to a whole species, 

 is sometimes called " racial." 



The occurrence of immunity from a second attack of an 

 infectious disease has given rise to numerous hypotheses in 

 the past. One theory supposed that after an attack of the 

 disease certain bacterial products are retained within the 

 body which prevent a second invasion. Another theory 

 supposed that the attack of the disease exhausts the supply 

 of some substance necessary for the growth of the microbes, 

 as plants sometimes exhaust the soil they grow in. 



It will be best to deal first with the results of some of the 

 experimental attempts to produce immunity. The more 

 recent theories will then be considered. 



Small-pox and Vaccination. The origin of vaccination 

 against small-pox with the virus of cow-pox has been de- 

 scribed in the historical sketch (p. 21). The nature of the 

 protection furnished by this virus has been the subject of 

 much controversy. The opinion of the present day inclines 

 to regarding vaccinia as small-pox which has been modified 

 by passage through a relatively insusceptible animal. Cer- 

 tainly there are many analogies between the protection 

 against small-pox afforded by vaccination and the other 

 examples of artificial immunity mentioned below. 



This question cannot be settled with certainty until the 

 organisms causing small-pox and vaccinia have been isolated 

 in pure culture. Their identity and mode of action may then 

 be determined. 



Small-pox has been inoculated into calves and passed 

 through other calves in succession, producing finally an 

 eruption indistinguishable from cow-pox. Lymph taken 

 from such calves has been used successfully to vaccinate 

 children. Not only does cow-pox protect against small-pox, 



