48 
M. PASTEUR. 
will give rise to the disease, but not fatally twenty, times out of 
twenty observations. These facts have an importance easily to be 
understood; they allow us to judge so far (of the disease which oc¬ 
cupies us) the problem of the possibility of a second attack. Let us 
take forty hens, and inoculate twenty with very virulent virus; 
they will die. Let us inoculate the other twenty with weakened 
virus ; all will be sick, but will not die. Let us allow these to 
get well, and inoculate them with very infectious virus; this time 
it will not kill them. The conclusion is evident; the disease pro¬ 
tects itself. It has the character of the virulent diseases which 
do not attack an organism twice. But be not blinded by the sin¬ 
gularity of these results. All in it is not so new as might be at 
first supposed. They have, however, on an important point, a 
true and real novelty, which must be exposed. Before Jenner, 
(and he for a long time practiced this method,) as I have al- 
read} 7 said, people were variolized, that is, inoculated with variola, 
to be protected from it. To-day, in some countries, sheep are va¬ 
riolized to protect them from variola ; bovine are inoculated with 
pluero-pneumonia to be protected from that disease. Chicken 
cholera is another immunity of the same kind. It is a fact wor¬ 
thy of attention, but not new in principle. The true, real novelty 
of the preceding observations—novelty, which gives rise to much 
thought in relation to the nature of virus, is that it is a question 
in this case of a disease, whose virulent agent is a microscopic 
parasite, a living being, cultivable outside of the economy. The 
varioloid virus, that of vaccine, glanders, syphilis, &c., &c., are 
unknown in this special nature. The new virus is an animated 
being, and the disease it gives rise to has, with virulent diseases 
so called, this point (so far unknown in virulent diseases with 
microscopic parasites) : the character of protecting from a second 
attack.* Its existence throws, so to speak, a bridge between the 
ground proper of the virulent diseases with living virus, and that 
of the diseases with virus whose life has never been discov¬ 
ered. 
I would not wish you to believe that the facts occur with the 
* Non-recidivity. 
