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