That an animal disease was due to such a minute and filterable organism was first shown by Loeffler in 1898 for the foot and mouth disease of cattle.  This is one of the most infectious and easily communicable diseases.  The lesions of the disease take the form of blisters which form on the lips and feet and in the mouths of cattle, and inoculation with minute quantities of the fluid in the blisters produces the disease.  Loeffler filtered the fluid through porcelain filters, hoping to obtain a material which inoculated into other cattle would render them immune, and to his surprise found that the typical disease was produced by inoculating with the filtrate.  Naturally the first idea was that the disease was caused by some soluble poison and not by a living organism, but this was disproved in a number of ways.  The most powerful poison known is obtained from cultures of the tetanus bacillus of which 0.000,000,1 of a gram (one gram is 15.43 grains) kills a mouse, or one gram kills ten million mice.  Loeffler found that 1/30 gram of the contents of the vesicles killed a calf of two hundred kilograms weight, and assuming that the essential poison was present in the fluid in one part to five hundred it would be several hundred times more powerful than the tetanus poison.  Further, the disease produced by inoculation of the filtrate was itself inoculable and could be transmitted from animal to animal.  It was also found that when the virus was filtered several times it ceased to be inoculable, showing that each time the fluid was passed through the filter some of the minute organisms contained in it were held back.

It is not known whether these organisms belong to the bacteria or protozoa, and naturally nothing is known as to their form, size and structure.  Up to the present about twenty diseases are known to be due to a filterable virus, and among these are some of the most important for animals and for man.  Among the human diseases, yellow fever, poliomyelitis, and dengue are so produced; of the animal diseases in addition to foot and mouth disease, pleuropneumonia, cattle plague, African horse sickness, several diseases of fowls and the mosaic disease of the tobacco plant have all been shown to be due to a filterable virus.  Of these organisms the largest is that which produces pleuropneumonia in cattle, and this alone has been cultivated.  It gives a slight opacity to the culture fluids, and when magnified two thousand diameters appears as a minute spiral or round or stellate organism having a variety of forms.  Its size is such that it passes the coarse, but is held back by the finer, filters and it is possible that this does not belong to the same class with the others.[1] The diseases produced by the filterable viruses taken as a class show much similarity.  They run an acute course, are severe, and the immunity produced by the attack endures for a long time.