UPON VIRULENT DISEASES-CHICKEN CHOLERA. 
49 
plainness and mathematical regularity that I speak of. It would 
be overlooking all the variability that exists in the constitutions of 
animals taken at hazard from a mass of domesticated individuals, 
and in the manifestations of life in general. No; the very viru¬ 
lent virus of chicken cholera does not always kill twenty times 
out of twenty ; but in the facts which I have seen, i f has done so 
in the minimum eighteen times, when not in twenty. And the 
weakened virus does not always protect twenty times out of 
twenty. In my observations it has been eighteen, and once six¬ 
teen. A single inoculation is not absolutely always protective. 
A more certain result can be looked for with two. 
If we compare these results with the great fact of vaccine 
with small pox, we will recognize that the weak microbe which 
does not kill, acts as a vaccine in relation to the microbe which 
kills; giving rise, in fact, to a decrease which may be called even 
mild, and which may protect from one which might be fatal. 
What would be necessary to convert this microbe of weakened 
virus into a true vaccine like that of cow pox? It is, if I may 
so express it, that it should be tixed in its special variety, and 
that to use it it would not be necessary to always look for it in its 
state of original vigor and purity. In other words, we are placed 
in the same doubt as that which for some time occupied Jenner’s 
mind. When he had demonstrated that inoculated cow-pox 
would protect from variola, he thought it would always be neces¬ 
sary to look for the cow-pox of the cow. We are at present at 
the same point in relation to chicken cholera, with however this 
great difference, that we know that our vaccine is a living being. 
Jenner soon found that he could do without the pox of the cow. 
We may do as well by carrying out microbsefrom culture to cul¬ 
ture. Will it then return to a very active virulency, or will this 
remain limited ? Astonishing as they may seem, facts bring us 
to the second supposition. At least in the small number of suc¬ 
cessive cultures that we have made, the virulency has not in¬ 
creased, and consequently we may believe that we have to deal 
with a true vaccine. More than that, one or two experiments arc 
favorable to the idea that the weakened virus remains such 
in passing in the organism of guinea pigs. Would it be so after 
