VACCINATION 
147 
is rubbed some of the virus secured from previous 
cases. After a few days this surface furnishes a yel¬ 
lowish, watery material which contains the protective 
stuff. This is gathered and mixed with glycerine, and, 
after careful tests of its purity, is distributed to physi¬ 
cians in small sealed glass tubes. This virus rubbed 
on to a scratched surface of the human skin induces 
a slight sore, sometimes accompanied by a little ma^ 
laise, and then heals. 
By this process the liability to smallpox is very 
greatly diminished, but the protection is reduced as 
time passes, so that revaccination is necessary if the 
fullest protection is to be secured. 
It is certain that smallpox is an infectious disease 
induced by some form of micro-organism. .But the 
exact character of this is still unknown. Attempts to 
cultivate it have thus far failed. It appears that the 
unknown organism suffers diminution in virulence by 
passing through the body of the relatively insuscepti¬ 
ble calf, and in this condition, while incapable of in¬ 
citing smallpox in man, is still potent to establish 
immunity. 
A good deal of opposition has developed here and 
there to vaccination even in recent times. This has 
been based partly upon the fear lest foreign and 
noxious material should be introduced into the body 
along with the virus. But if it be carefully prepared, 
this fear is groundless. While accidents are not im¬ 
possible, the ill effects which now and then appear are 
