FOURTH LECTURE. 



THE RELATION OF MICRO-ORGANISMS TO THE 

 PRODUCTION OF DISEASE. 



One who has followed closely the experimentation on the 

 relation of micro-organisms to the production of disease, will, 

 I think, accept the following propositions as a basis for a 

 farther study of the subject: 



1st. Experiment has shown that there are many micro- 

 organisms which do not produce disease or injury; that the 

 mere presence of these life forms is not, in itself, a sufficient 

 cause of disease; but that there are certain micro-organisms 

 that uniformly induce disease when they are planted in, or 

 obtain entrance to, the bodies of animals or men. 



2d. All through the experimentation that has been had on 

 this subject, it has been noticed that poisons have originated 

 in connection with the disease-producing organisms. 



3d. These poisons seem to be a product of the organisms, 

 and, while produced only in connection with the life and 

 growth of the microscopic plants, may be separated from 

 them ; and, when so separated, produce direct poisoning 

 similar to that induced by the poisons from the higher plants. 



4th. This poisoning is different from the disease that is 

 induced by the organisms, in that it is direct, and compara- 

 tively immediate; while infectious disease arises only after a 

 more or less definite stage of incubation. 



5th. Therefore, it is probable that each infectious disease is 

 caused by a micro-organism that is capable of developing in 

 the tissues, or blood, and forming poisons through changes 

 wrought in the molecular forms of matter, by virtue of its 



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