When the coarser of such filters have been long in 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.