502 SMALLPOX AND VACCINATION. 



course be the isolation of the same micro-organism from 

 both, and the obtaining of all the results just detailed by 

 means of pure cultures or the products of such. In the 

 absence of this evidence we are at present justified in 

 considering that there is strong reason for believing that 

 vaccinia and variola are the same disease, and that the 

 differences between them result from the relative sus- 

 ceptibilities of the two species of animals in which they 

 naturally occur. 



With regard to the relation of cowpox to horsepox, it is 

 extremely probable that they are the same disease. Some 

 epidemics of the former have originated from the horse, 

 but in other cases such a source has not been traced. 

 Cattle plague from the clinical standpoint, and also from 

 that of pathological anatomy, resembles very closely human 

 smallpox. Though each of the two diseases is extremely 

 infectious to its appropriate animal, there is no record of 

 cattle plague giving rise to smallpox in man or vice versa. 

 When matter from a cattle plague pustule is inoculated in 

 man, a pustule resembling a vaccine pustule occurs, and 

 further, the individual is asserted to be now immune to 

 vaccination ; but vaccination of cattle with cowpox lymph 

 offers no protection against cattle plague, though some have 

 looked on the latter as merely a malignant cowpox. 

 Sheep-pox also has many clinical and pathological analogies 

 with human smallpox, and facts as to its relation to cowpox 

 vaccination similar to those observed in cattle plague, have 

 been reported. Smallpox, cowpox, cattle plague, horse- 

 pox, and sheep-pox, in short, constitute an interesting 

 group of analogous diseases, of the true relationships of 

 which to one another we are, however, still ignorant. 



Micro-organisms associated with Smallpox. Burdon 

 Sanderson was among the first to show that in vaccine 

 lymph there were certain bodies which he recognised as 

 bacteria. Since then numerous observations have been 

 made as to the occurrence of such in matter derived from 

 variolous and vaccine pustules. In especially the later 

 stages of the latter, many of the pyogenic organisms are 



