156 PASTEUR AND HYDROPHOBIA III 



explain the results obtained by Pasteur in the cure of 

 hydrophobia. 



The general fact that the ill effects of some diseases 

 due to specific virus or poisons can be averted by in- 

 oculating a patient with the virus in a modified con- 

 dition — as, for instance, when vaccination is used as a 

 preventive of small-pox in man — may be explained 

 more or less satisfactorily by three different supposi- 

 tions. The first supposition is that the virus is a living 

 matter which grows and feeds when introduced into 

 the body of the inoculated animal, and that it exhausts 

 the 50^7^that is to say, uses up something in the blood 

 necessary for the growth of the virus ; accordingly, 

 when the soil has been exhausted by a modified and 

 mild variety of the virus, there is no opportunity for 

 the more deadly virus, when it gains access, to feed and 

 multiply. A second supposition is that the virus does 

 not exhaust the soil, but as it grows in the animal 

 body produces substances which are poisonous to itself, 

 and these substances, remaining in the body after they 

 have been formed there by a modified virus, act poison- 

 ously upon the more deadly virus when that gains 

 access, and either stop its development altogether or 

 greatly hinder it. An analogy in favour of this suppo- 

 sition is seen in the yeast plant, which produces alcohol 

 in saccharine solutions until a limited percentage of 

 alcohol is present, then the alcohol acts as a poison to 

 the yeast plant, and neither it nor any other yeast 

 plant of the kind can grow further in that solution. 

 A third supposition is that, whether the virus be a 

 living thing or not, the protective result obtained by 



