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.