eT tee a Meee noon 
B45 
several animals with the same sample of virus. If the majority of the 
animals yielded atypical or abortive lesions, we should be warranted in 
saying that the virus was avirulent. If, on the other hand, a majority 
of the animals gave typical lesions, we should classify the virus as 
virulent. 
Applying this test to all our experiments, we find that certain strains 
of variola virus we have used may be classified as virulent and others 
as avirulent. We may also say that the transfer of a strain of variola 
virus from one monkey to another tends to reduce its virulence. It 
also seems to be the case that drying, either in the course of the disease, 
as in the formation of the crust, or artificially, reduces the virulence. 
In the same way, exposure to 60 per cent glycerin lowers the virulence. 
This brings up the question of the nature of the difference between 
variola and vaccine virus. The two strains of virus may be said to bear 
a relation of virulence and avirulence to one another, so far as the results 
of our experiments go. However, there is an important difference 
between the two contagiums which we have not been able to bring out 
experimentally but which is a matter of common knowledge. This 
difference may be put as follows: Vaccine produces a local lesion at 
the site of inoculation, but no exanthem, and is not air-borne; variola 
produces a local lesion at the site of inoculation, an exanthem, and is 
air-borne. 
The occurrence of an exanthem and of an air-borne contagium still 
needs explanation. In the extenuation of a strain of variola virus we 
see that the exanthem-producing potentiality is lost before the virus 
becomes entirely inactive. Variola virus which has lost its power to 
produce an exanthem has practically no points of difference from a 
vaccine virus, so far as its reactions on the monkey are concerned. 
Whether or not it has in fact become a vaccine virus could only be 
determined by inoculations and by exposure experiments with human 
beings. Such procedures are of course impossible and the solution of 
the problem must await the findings of an experimental animal in 
which variola vera can be produced under the same conditions which 
produce the disease in man. 
CONCLUSIONS. 
(1) Variola virus is less resistant to desiccation than vaccine virus. 
(2) Variola virus does not pass through the “N” Berkefeld filter. 
(3) Variola virus is attenuated by long exposure to 60 per cent 
glycerin. The virus so treated loses its power to produce an exanthem 
when inoculated on the skin of the monkey (M. cynomologus). 
(4) Variola virus tends to die out when passed repeatedly through the 
monkey. The exanthem-producing power is lost before the virus has 
become incapable of producing a primary lesion. 
