ESSAYS ON BACTERIOLOGY. 51 



immunity. This work has not been without striking 

 results. There are several diseases, such as anthrax, 

 symptomatic anthrax, swine erysipelas, and, perhaps, 

 tetanus, rabies and diphtheria, the attack upon which 

 has shown an encouraging degree of success. 



It will be seen that, as a practical matter, the imi- 

 tation of natural self -limitation and immunity, that is, 

 cutting short or curing, and protecting against infec- 

 tion, present quite different problems. 



Here rises into prominence another matter. As al- 

 ready stated, bacterial growth is often or usually ac- 

 companied by the formation of several new com- 

 pounds. Which of these is the effective agent in the 

 self -limitation, and which in affording immunity? Are 

 the substances which do the harm and render the dis- 

 ease dangerous the same as those which do the good? 

 This is a question of great importance, especially when 

 we come to the problem of imitating self-limitation 

 and thereby cutting short and curing a disease. If 

 the harmful and helpful agents are the same, it is open 

 to question whether we may legitimately introduce 

 into a body already suffering under an active disease 

 an agent which, while re-enforcing the tendency to 

 self-cure, may also increase the tendency to death. 

 Under these conditions it would seem that such treat- 

 ment could, at best, be of value only in the earliest 

 stage of the disease, and in this way only was it that 

 Koch hoped to do good by the use of his "lymph" 

 against tuberculosis. If the harmful and helpful 



