PROTECTIVE INOCULATIONS. 289 
readily accounts for the spread of the disease in a poultry yard when 
a case occurs. 
In the same communication Pasteur records his observation that 
“by a certain change in the method of cultivation the infectious mi- 
crobe may be caused to have a diminished virulence.” Also the fact 
that fowls inoculated with this “attenuated” virus recover and are 
subsequently immune against infection by the most virulent microbes. 
In concluding this communication Pasteur says: 
“Tt appears to be superfluous to point out the principal result of the facts 
which I have had the honor to present to the Academy. There are two, how- 
ever, which it may be useful to mention. These are, first, the hope of ob- 
taining artificial cultures of all kinds of virus; second, the idea of seeking 
for virus vaccines of the virulent maladies which have devastated so often, 
and still devastate, the human race, and are such a scourge to that branch of 
agriculture which relates to the breeding of domestic animals.” 
In his communication of October 26th, 1880, Pasteur gives his rea- 
sons for concluding that attenuation of virulence is due to the action 
upon the microbe of atmospheric oxygen. He infers this from the 
fact, demonstrated by experiment, that when cultures are placed in 
hermetically sealed tubes, from which the oxygen present is soon ex- 
hausted by the growth of the microbe, they do not become attenuated 
in virulence; whereas cultures which are freely exposed to the air 
gradually become attenuated. Pasteur sees in this an important fact 
bearing upon the explanation of the natural extinction of epidemics. 
He says: 
‘‘May we not suppose, then, that it is to this influence that we must at- 
tribute, in the present as in the past, the limitation of great epidemics ?” 
In his communication to the French Academy, made on February 
28th, 1881, Pasteur treats of the attenuation of virulence by the method 
above referred to and by the method of Toussaint, and also of the re- 
establishment of the virulence of attenuated cultures. He says: 
‘¢The secret of the return to virulence rests solely, at present, upon suc- 
cessive cultures in the bodies of certain animals.” 
Thus he had found by experiment that the anthrax bacillus might 
be so attenuated that it was harmless for grown guinea-pigs, or even 
for guinea-pigs a month or a week old, but it would still kill guinea- 
pigs just born—a day old. By inoculating an older pig with the blood 
of this one, and so on, the virulence was gradually augmented, until 
finally a virus might be obtained which would kill adult animals, and 
even sheep. In the same way the attenuated microbe of fowl cholera 
could be restored to virulence by first inoculating small birds, such as 
sparrows or canaries. 
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