BACTERIAL VACCINES 189 



anthrax; or by passing the virus through a less sus- 

 ceptible animal, as is done in vaccination against small- 

 pox; or by means of chemicals such as the addition 

 of iodine solution to diphtheria toxin, as was formerly 

 done by Behring; or by means of heat, as was also 

 formerly done with diphtheria toxin. 



(2) By means of dead cultures of the virus. The 

 cultures can be killed either by heat or by the use 

 of chemicals. 



(3) By the so-called " combined method," i.e., 

 by first administering a dose of the specific immune 

 serum and subsequently the virus. This method 

 has been used in typhoid fever, cholera, and plague. 



(4) By means of the products of autolysis of the 

 cultures. This has also been used in typhoid fever, 

 and seems to possess certain advantages over the 

 use of native cultures. 



(5) By means of various combinations of the 

 preceding methods. 



The choice of these various methods of immuni- 

 zation depends on the nature of the infecting virus. 

 With some infections dead cultures apparently are 

 able to cause the production of full protective pow- 

 ers, while in other infections the body seems to 

 require a greater stimulus. In these, the use of 

 attenuated living cultures may bring about the 

 desired immunity. Finally there are infections 

 in which nothing short of fully virulent cultures 

 seems to bring about the development of sufficient 



