86 BACTERIOLOGY. 
ditions will not produce disease. As already men- 
tioned, bacteria even under similar conditions differ 
enormously in the amount of poison which each organ- 
ism produces and in their ability after gaining entrance 
to multiply in the body. 
To understand at all the production of disease through 
bacteria we must recognize that both the body invaded 
and the bacteria which invade are living organisms. 
They are in bulk, wide apart, but both have life. Just 
as there are different races and species of animals, there 
are different races and species among bacteria, and just 
as the descendants of one animal species under changing 
conditions gradually become diverse, so do the descend- 
ants of one bacterial species. Considering these facts, 
we can readily understand how all of bacteria do not 
grow equally well in every variety of animal, nor even 
find the body of the same animal always equally suit- 
able. This is all the more apparent when we consider 
that the study of bacteria in the more simple and 
known conditions of artificial culture media has already 
shown us how extremely sensitive many bacteria are to 
slight chemical, thermal, and other changes. 
Thus if we take specimens of diphtheria bacilli from 
three different cases of diphtheria, we find that on grow- 
ing them for several days in suitable bouillon one will 
have produced poison in the culture fluid to such a 
degree that one drop suffices to kill a large guinea- 
pig; the second, grown in a similar manner, will kill 
another animal of the same size with half a drop; 
while the third will kill with one-tenth of a drop. 
In other words, different varieties of diphtheria bacilli 
under similar conditions have different toxin-producing 
powers. 
