a244 SUSCEPTIBILITY AND IMMUNITY. 
demic prevalence of this disease would show that second attacks are 
more common than is indicated by these figures. 
That immunity may result from a comparatively mild attack as 
well as from a severe one is a matter of common observation in the 
case of small-pox, scarlet fever, yellow fever, etc.; and since the dis- 
covery of Jenner we have in vaccination a simple method of produc- 
ing immunity in the first-mentioned disease. Theacquired immunity 
resulting from vaccination is not, however, as complete or as per- 
manent as that which results from an attack of the disease. 
These general facts relating to acquired immunity from infectious 
diseases constituted the principal portion of our knowledge with re- 
ference to this important matter up to the time that Pasteur (1880) 
demonstrated that in the disease of fowls known as chicken cholera, 
which he had proved to be due to a specific microédrganism, a mild 
attack followed by immunity may be induced by inoculation with an 
“* attenuated virus *’—?.e., by inoculation with a culture of the patho- 
genic microérganism the virulence of which had been so modified 
that it gave rise toa comparatively mild attack of the disease in 
question. Pasteur’s original method of obtaining an attenuated virus 
consisted in exposing his cultures for a considerable time to the ac- 
tion of atmospheric oxygen. It has since been ascertained that the 
same result is obtained with greater certainty by exposing cultures 
for a given time to a temperature slightly below that which would 
destroy the vitality of the pathogenic microérganism, and also by ex- 
posure to the action of certain chemical agents. 
Pasteur at once comprehended the importance of his discovery, 
and inferred that what was true of one infectious germ disease was 
likely to be true of others. Subsequent researches, by this savant 
and by other bacteriologists, have justified this anticipation, and the 
demonstration has already been made for a considerable number of 
similar diseases—anthrax, symptomatic anthrax, rouget. 
A virus which has been attenuated artificially—by heat, for ex- 
ample—may be cultivated through successive generations without re- 
gaining its original virulence. As this virulence depends, to a con- 
siderable extent at least, upon the formation of toxic products during 
the development of the pathogenic microorganism, we naturally infer 
that diminished virulence is due to a diminished production of these 
toxic substances. 
There is reason to believe that a natural attenuation of virulence 
may occur in pathogenic bacteria which are able to lead a sapro- 
phytic existence during their multiplication external to the bodies of 
living animals, and the comparatively mild character of some epi- 
demics is probably due to this fact. 
