INFECTION, SUSCEPTIBILITY, AND IMMUNITY 25 



symptomatic anthrax, and hen cholera, while all animals 

 except man and monkeys are immune to syphilis. 



Such immunity is termed natural immunity in contra- 

 distinction to acquired immunity, which is exemplified 

 in the immunity which follows an attack of some infec- 

 tious disease, as measles, scarlet fever, and whooping- 

 cough, and may also be produced artificially by vacci- 

 nation, injection of antitoxin, etc. 



Vaccination. In vaccination the subject is infected 

 with an attenuated or modified form of the disease 

 which, however, is sufficient to give immunity to further 

 attacks. Jenner, by infecting individuals with cow-pox, 

 which is a modified form of small-pox caused by the 

 growth of the small-pox germ on an unfavorable host, 

 the cow, procured immunity to small-pox. Pasteur 

 produced a vaccine against anthrax by subjecting an 

 anthrax culture to high temperature for twenty days, 

 thus so destroying its virulence that an animal infected 

 with such a culture had a very mild attack of anthrax, 

 which, however, was sufficient to render it immune to 

 further attacks of anthrax. 



Bacteriolysis. It has been found that normal blood- 

 serum possesses the power to destroy bacteria under 

 certain conditions. This property is termed bacterio- 

 lysis. Various substances in the serum are concerned 

 in such destruction, such as alexins, lysins, etc. Just 

 what part each and every one of these substances play 

 in bacteriolysis is not as yet definitely known. 



