The causative agent of hydrophobia has been sought for most thoroughly, but, so far, probably in vain. It was the fruitlessness of his search for the organism of hydrophobia that led Pasteur to suggest that it might be due to an organism too small to be seen, even with the highest and best microscopes. It is generally stated, however, that the cause of hydrophobia does not pass through ordinary bacteria-proof filters which permit the ultramicroscopic viruses of several other diseases of unknown etiology to go through. Whether the non-filterability of the virus of hydrophobia is due to the large size of the elements to form union with animal cells, or other causes, can not now be stated.
In these days, when discoveries of protozoan organisams in diseases of unknown etiology are made with startling frequency; when, if everything claimed comes true, the causes of smallpox, of vaccinia, of yellow fever,