Whereas the real cause is an invisible filterable virus.
"Disturbing Sun"
Robert Shirley Richardson
The specific cause of foot-and-mouth disease is a filterable virus that is present in the serum from the vesicles, the saliva, milk, and various body secretions and excretions from the sick animal.
"Common Diseases of Farm Animals"
R. A. Craig, D. V. M.
It will pass through the Berkfelt filter, which is the most minute filter known to science, and is therefore known as a filterable virus.
"The Veterinarian"
Chas. J. Korinek