76 THE RELATION OF BACTERIA TO DISEASE 



cause true general smallpox. The great Pasteur found 

 that if he heated anthrax bacilli and injected them 

 into sheep, these animals became resistant to the disease 

 anthrax. Since the time of Pasteur the management 

 of the process which has been called " active immuniza- 

 tion" has been learned. To accomplish this a virus 

 must be treated as was the virus of smallpox, that 

 is, it must be rendered incapable of causing general 

 disease, but it must not be so altered that it has no 

 relation to its original form. The living organisms 

 can be taken and subjected to higher or lower tempera- 

 tures than those preferred by the individual species, or 

 they may be injected into animals until they will 

 merely live without producing disease. This is called 

 reducing virulence. They may be killed by heat or 

 obtained in mass and crushed and ground into a pulp. 

 Again, the broth or other material upon which they 

 grow may be used after removing the bacterial bodies 

 by filtering them off through porcelain filters. Having 

 obtained the virus in a reduced state either dead or as 

 active principles, it is injected beneath the skin of the 

 individuals whom it is desired to protect, beginning 

 in minute doses and increasing the quantity as the 

 condition permits. By this means the resistance of 

 the animal or person to this particular germ is increased, 

 and the process corresponds to that of the production 

 of antitoxin in horses, that is, making an antipoison, 

 or, as it is called, an antibody. The method just 

 described is usually reserved for the bacteria which 

 produce intracellular or endotoxins. The method has 

 been used in treating anthrax, typhoid, cholera, etc. 



