The virus is in reality pathogenic for these three kinds of rats when 
ingested. No special difference was noted in its effects upon the various 
species. 
As the work progressed it soon became evident to me that the result 
depended largely upon the amount of the culture ingested. By starv- 
ing rats for a day or two and then giving them all they could be induced 
to eat and drink of the cultures, a very positive result was obtained. In 
one instance of 27 rats so fed, all died within a w*eek. If the rats are 
given a small amount the effect is uncertain — only a few die. In one 
instance I fed 70 rats wdth 4 agar tubes, and only 7 died. Upon refeed- 
ing them with very large quantities, 9 more died. The survivors were 
then fed with all they could be induced to eat every day for a week, 
without effect. 
It, therefore, seems plain that a large primary dose pioves fatal, and 
a small dose is not only uncertain, but produces an immunity. This is 
a very important factor, for it is likely that in the wild state rats would 
often partake of an amount too small to cause death. Such rats may 
then subsequently eat large amounts of the culture with impunity. 
It would seem, then, that after all, the virus is not so different from 
the laying out of a chemical poison, depending as it does for its effect 
upon the amount ingested. A chemical poison, however, does not 
l) 0 ssess the disadvantage of producing an immunity. Another disad- 
vantage possessed by the virus is the rapid deterioration in virulence 
which occurs when it is exposed to the action of air and light, or when 
it becomes dry, as is very apt to happen when laid out for rats in the 
wild state. 
Against this the virus has the very decided advantage over the usual 
chemical agents used to kill rats that in so far as known it is harmless 
for man and domestic animals. 
THE DISEASE PRODUCED BY THE VIRUS HAS FEEBLE POWERS OF 
PROPAGATING ITSELF AMONG RATS. 
Danyz has pointed out the fact that the epidemic produced by his 
virus in rats is self limiting. This is due to two causes : 
1. The attenuation of the virus. 
2. The resistance of the rats. 
That the disease can be spread from rat to rat by eating each other 
can not be doubted, but this method of the spread of the infection is 
very feeble. I fed rats upon the organs (spleen, liver, etc.), of other 
rats dead of the infection, without producing any eflect. Bats will some- 
times eat each other, but sometimes they refuse. I fed dead and sick 
rats to 10 healthy rats in a cage, and although food was withheld for four 
days they refused to touch the dead bodies. On other occasions the 
rats were very vicious, fighting and killing the sick and feeble and eat- 
ing the dead bodies. 
