VIRULENCE AND ATTENUATION 305 



bustion of the tissues in the absence of food, and conse- 

 quently as the agent of enervation, is, at the same time, 

 an agent of attenuation. Attenuation and weakening 

 are synonymous, and we have here a conception which 

 harmonizes well with our idea of strife in the microbial 

 diseases. That which is harmful to the microbe is of 

 value to its host. 



We are then justified in asking ourselves if all these 

 causes of weakening on the part of the microbe, all these 

 factors which contribute more or less quickly to its 

 death, do not first cause it to pass through a series of suc- 

 cessive attenuations, that is, transform it into vaccines. 

 To this new question, experiment replies without hesi- 

 tation, "Yes." In a general way, attenuation is one of 

 the forms of the gradual weakening of a microbial cell 

 which is on its way to death, and every action harmful 

 to the microbe begins by diminishing its virulence. 

 Such, for example, is heat, too high a degree of which 

 kills the microbe, as we know. Between the optimum 

 temperature for culture and the death point exists a 

 zone of attenuation, observed by M. Toussaint and 

 carefully studied by M. Chauveau, for the anthrax 

 bacteridium. The duration of the heating should be in 

 inverse ratio to the elevation of the temperature and, for 

 a given temperature, directly proportional to the degree 

 of attenuation to be obtained. 



Next to the action of heat naturally comes that of the 

 light of the sun. It kills the microbe after a certain 

 length of exposure to it, but before killing, it causes 

 attenuation. This is the conclusion from my experi- 

 ments, followed by those of M. Arloing. 



So much for the physical agents. Now for the 

 chemical ones. Oxygen is a physiological factor of 

 the greatest importance, and we have already examined 

 its role in this relation. But it plays also a role more 



