ETIOLOGY 15 
ultra-visible virus, the exact nature of which has not yet 
been fully determined. 
That these germ-free fluids will, when inoculated, set 
up a disease corresponding very closely to distemper 
seems undoubted, and it was the realisation of this fact 
which first prompted many bacteriologists to attempt to 
establish a method of conferring an artificial immunity in 
the healthy by experimentally inducing a benign attack. 
Unfortunately they were not very successful. 
Fic. 1.—BacILLuS BRONCHISEPTICUsS. 
Isolated by Ferry, and said to be the cause of distemper. 
Magnified 1,000 diameters, , 
Bacterial Theory.—Numerous investigators have from 
time to time isolated various micro-organisms which they 
have considered to be the specific cause of distemper, 
but none appear to have been able to convince the 
veterinary medical world that they have: solved the 
problem (see note on Bacteriology). 
Ferry's Bacillus Bronchisepticus.—However, in October, 
1908, after four years of continual experimental work, 
