INFECTION AND IMMUNITY. 37 



by smallpox even though they were exposed to it. 

 Jenner reasoned that if an accidental infection with 

 cow-pox could prevent against smallpox it would be a 

 rational procedure to purposely infect with cow-pox. 

 So, acting on the advice of his patron, Dr. John 

 Hunter, he inoculated a boy with pus from a cow-pox 

 pustule in May, 1796, and two months later injected 

 the pus from a smallpox pustule without producing 

 any disease. 



When immunity is acquired by introducing into 

 the body the infectious agents in modified form or in 

 small amount, it is spoken of as active immunity be- 

 cause the body tissues take an active part in forming 

 the substances that give protection. Our knowledge 

 of how immunity is produced in this way is due prin- 

 cipally to Pasteur, who' found that the bacteria pro- 

 ducing cholera among fowls became much less virulent 

 after being cultivated for long periods of time on 

 artificial culture media or after cultivation at increased 

 temperatures. By injecting gradually increasing 

 amounts of these attenuated bacteria of chicken- 

 cholera into fowls he was able to immunize them to 

 the disease. 



The introduction of dead bacteria or vaccines in vaccines 

 increasing doses is often used to develop immunity 

 against those bacteria whose poisons are intracellular. 

 This method has been practised a great deal these last 

 few years, and has been attended with considerable 

 success in some infections. Its most successful appli- 



