44 



I am alluding lies not in connection with the 

 bacillus itself, but with the condition of the 

 medium upon which it may chance to fall. It 

 has been found that the parasite requires virgin 

 soil for its growth. This observation stands in 

 harmony with the result of common experience 

 as regards disposition to contract infectious 

 disease. It has been from remote times generally 

 known that a person who has passed through one 

 attack of an infectious disorder is not liable to 

 the same extent as before to become affected on 

 exposure to contagion. An influence has been 

 exerted giving rise to more or less protection 

 being afforded against a recurrence of the disease. 

 Now, it happens that by certain means the bacillus 

 may be brought into such a weakened state as only 

 to occasion, when introduced into the system of an 

 animal, an effect of a mild nature, not dangerous 

 to life, instead of the ordinary form of disease ; but 

 the effect produced, and this is the great point 

 of practical importance, is as protective against a 

 subsequent attack as the fully developed disease. 

 There are two methods by which attenuation 



