There is a number of transmissible diseases, both human and animal, some of them exquisitely contagious, the causes of which are still unknown, notwithstanding the most careful search. Of the human diseases belonging to this group I would mention especially scarlet fever, measles, chicken-pox, yellow fever, typhus fever and hydrophobia. Smallpox and vaccinia may also be left here for the present. Of the animal diseases of this group the most important are foot and mouth diseases, peripneumonia, bovine pest, sheep-pox (clavelée), chicken-typhus or chicken-pest, "horse sickness" and epithelioma contagiosum of fowls. The "mosaic disease" of the tobacco plant ("Mosaik oder Fleckenkrankheit des Tabaks") may be given as an example of a contagious plant disease of unknown etiology.
THE INFECTIOUS NATURE OF THE DISEASES DISCUSSED.
In discussing the probable nature of diseases of this kind perhaps the first question to arise will be, "What evidence is there that they are caused
HEKTOEN L. RECENT INVESTIGATIONS BEARING ON INFECTIOUS DISEASES OF UNKNOWN ETIOLOGY.. JAMA. 1903;XLI(7):405-409. doi:10.1001/jama.1903.04480040001001