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. 
