XIII ATTENUATION OF VIRUS OF FOWL ENTERITIS in 



fowl against a second attack ? It must be evident 

 that unless this is the case, that is, unless an animal 

 becomes refractory to a second attack by having passed 

 through a first mild attack, a primary inoculation with 

 attenuated virus would be useless ; it is for this reason 

 imperative to ascertain whether, in a particular in- 

 fectious disease, an animal having had the disease in a 

 mild form becomes thereby possessed of immunity 

 against a second attack. We have mentioned in a 

 previous chapter experiments (Experiment 4) in which 

 fowls had been the subject of inoculation with cultiva- 

 tions of the bacillus of fowl enteritis, and had become 

 ill but had recovered. In addition to these fowls 

 (three in number) five more fowls had recovered from 

 the disease after inoculation with culture — these five 

 animals will be mentioned below in connection with 

 other series of experiments — so that there were eight 

 fowls that had passed through an attack of the disease 

 but had recovered. When they had seemingly quite 

 recovered — they were lively, fed well, and appeared 

 in all other respects well — they were subjected to a 

 second inoculation, some with considerable doses of a 

 recent first broth sub-culture, that is, a broth sub-culture 

 made from a recent gelatine culture of the heart's 

 blood of a fowl dead of the disease ; others with blood 

 or spleen tissue of a fowl dead of the disease. The 

 same broth culture was used at the same time for in- 



