18 APPLIED BACTERIOLOGY 



of the cases of which we frequently hear of individuals, or 

 even families, being poisoned by partaking of some particular 

 meat, fish, or other food, that has had the opportunity of 

 undergoing partial decomposition. These cases are in- 

 variably due to the fact of the food in question being in an 

 unsound condition, whereby it contained organisms which 

 generated the poison ; and even though the bacteria may 

 have been destroyed during the process of cooking, the 

 toxic substance remains in the food, to produce the most 

 disastrous effects on its being eaten. 



Some pathogenic organisms, which under ordinary cir- 

 cumstances cause disease and death, can by proper methods 

 be so modified in their properties that they can be made to 

 serve as antidotes to the very diseases they cause. This 

 discovery, which was due to the genius of Pasteur, is the 

 greatest romance of modern science ; it has opened a new 

 epoch in the annals of medicine, and has revolutionized the 

 treatment of disease. For example, the bacillus of anthrax, 

 if cultivated at a temperature rather higher than blood-heat, 

 becomes no longer fatal when inoculated into animals, but 

 produces only a slight constitutional disturbance, after 

 which treatment the animals are found to be ' immune,' or 

 protected against the virulent form of anthrax. This 

 great principle of an 'attenuated' virus conferring immunity 

 is the basis of many systems of protective treatments 

 which are becoming of ever -increasing importance in the 

 conflict with infectious disease. A full account of the 

 theory and practice of these ' antitoxin ' treatments will be 

 found later. 



Variation of Bacteria. — The capacity of bacteria to 

 produce the manifestations described above is dependent 

 on the presence of suitable conditions, which in some cases 

 can be defined, and in others have yet to be worked out. 

 Thus it may happen that different races of the same 



