VIEULENT DISEASES. 183 



taminated subjects the slightest excoriation or scratch 

 of the skin is sufficient to allow the virus to enter. 

 When others besides the class that we have named 

 become infected, it is because they live in the neigh- 

 bourhood of herds smitten with splenic fever. There 

 are also certain flies which transport the virus. Sup- 

 pose one of these flies to have sucked the blood of an 

 animal which has died of splenic fever, a person 

 stung by that fly is forthwith inoculated with the 

 virus. 



At the very time (1850) when these first experi- 

 ments were being made by the Medical Association of 

 the Eure-et-Loir, Dr. Eayer, giving an account in the 

 ' Bulletin de la Societe de Biologie de Paris ' of the 

 researches he had made, with his colleague, Dr. 

 Davaine, on the contagion of splenic fever, wrote : 

 'In the blood are found little thread-like bodies 

 about twice the length of a blood corpuscle. These 

 little bodies exhibit no spontaneous motion.' 



This is the date of the first observation on the 

 presence of little parasitic bodies in splenic fever, but, 

 strange to say, no attention was paid to these minute 

 filaments. Kayer and Davaine also paid no atten- 

 tion to them. This indifference lasted for thirteen 

 years ; it would have lasted longer still, if the parasitic 

 origin of communicable diseases had not been brought 

 before the mind by each new publication of Pasteur's. 

 From 1857 to 1860 it will be remembered that he had 



