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 o-n 
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 
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- 
Active 
immunity 
Vaccines 
