SAFEGUARDS OF THE BODY 135 



In order to understand what has happened in the 

 body of a person who has thus acquired immunity 

 through a successfully weathered attack of an infec- 

 tious disease, it will be necessary for us to look at 

 some very remarkable achievements of the past few 

 years in the prevention and cure of diphtheria. For, 

 though the fact of immunity acquired through disease 

 has been known so long, no one until recently could 

 offer even a plausible conjecture as to the reason for it. 

 Among the earlier of the disease-inducing bacteria to 

 be discovered, some twenty years ago, was the bacillus 

 of diphtheria. This is a little rod-like plant found only 

 in connection with this disease, or in those who have 

 been exposed to it. It is readily cultivated in the' 

 laboratory, being very fond of beef tea, in which it is 

 commonly grown. 



When a few of these living bacilli from the culture 

 are put beneath the skin of animals, such as rabbits 

 or guinea pigs, a fatal disease is induced, essentially 

 similar to the disease diphtheria in man. 



In the early days of bacteriology it was believed 

 that, in order to induce artificially the symptoms of 

 an infectious disease, the living germs must be put 

 into the body, and grow there. But it was presently 

 discovered that if you separate all the germs from a 

 culture of the diphtheria bacillus, and introduce the 

 beef- tea in which they had grown for some time, into 

 an animal, you can induce the symptoms of the disease 

 just as well as if the germs themselves are put in. 



