58 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL. VIII. 
quitoes from the sick, 1.e., prevention of infection by mosquitoes, and ex- 
clusion of mosquitoes from the well,:7.e., prevention of infection by mos- 
quitoes; it has been possible to rid Havana of yellow fever ; a practical 
result in disease prevention which has never been surpassed. 
But the special interest which we have in this virus of yellow fever 
is in the fact that microscopic examination and culture methods failed to 
show organisms in the mosquitoes or in the blood during the first two days. 
It was natural then that the filtration test should be applied and with 
positive results it was shown that the virus passed freely through a Berke- 
feld filter which was impermeable for bacteria. 
It is quite possible that other human diseases may be found to be due 
to ultramicroscopic organisms but as far as some of the diseases of as yet” 
unknown etiology are concerned, this is not likely. It seems that rabies 
for instance is not filterable. It is held back by the Pasteur filter*. Simi- 
larly vaccine virus will not pass through the filter. In regard to other 
diseases we do not know anything definite. 
We may now ask ourselves what is the nature of these minute living 
particles? Are they simply smaller bacteria than we are accustomed to 
deal with, or are they a minute species of some type of animal parasite, 
or do they belong to a class of organism smaller and simpler than any- 
thing we have yet considered possible? 
These are difficult questions to answer, but in regard to the first, we 
may perhaps be a little more positive. 
I called attention at the beginning to the marked uniformity of size 
among the bacteria. That in itself is no proof that there may not be bac- 
teria many times more minute than those we are accustomed to, but it 
is a presumption against that view. A more important reason however 
is in the fact that we have not yet met with an ultramicroscopic organism 
of a non-pathogenic character. They are all disease producers and if there 
were saprophytic organisms of this character they would certainly have 
been found, because it is the commonest experiment in a laboratory to pass 
fluid through a Pasteur filter in order to sterilize them without heat. I 
only know of one observer who purposely filtered a large number of different 
putrefying mixtures, both animal and vegetable, with the hope of finding 
a saprophytic ultramicroscopic organism. - This was von Esmarch, and 
he failed entirely in a very large series of experiments, only finding a small 
spirillum which would pass fairly readily through the most permeable of the 
Pasteur filters. 
* Recent work seems to show that the virus of rabies can be filtered through the most permeable of 
these filters. 
