A considerable group of maladies is at present classed under the designation of virus diseases. Smallpox, vaccinia, yellow fever, rabies, poliomyelitis, herpes simplex, foot-and-mouth disease, fowl pox, fowl plague, canine distemper, and psittacosis are included. Although the exact nature of the pathogenic factors involved remains unknown, the large group of virus diseases is "loosely hung together mainly by the thread of filterability of the active agents which cause them." In his Harvey Lecture in 1929, Goodpasture1 remarked that it seems almost hopelessly confusing to one newly introduced to the subject of filtrable viruses to find such a formidable array of apparently unrelated or at most seemingly superficially related morbid entities grouped together nominally only on the basis of the fact that their active agents can, in some cases with ease and in others with great difficulty, be forced through the small pores of so-called antibacterial filters of various kinds.
VIRUSES. JAMA. 1932;99(6):479-480. doi:10.1001/jama.1932.02740580047012