domestic service in filtering drinking water, bacteria may grow in
and through them giving greater bacterial content to the supposed
bacteria-free filtrate than in the filtering water.

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