Bacteria , their Nature and Function. 
379 
In the fermentative processes studied by Pasteur and others it was 
shown that each specific fermentative process is due to the growth 
and multiplication of a specific microbe. Just the same is the case 
with the infectious diseases — when from a substance which is in the 
process of fermentation, a trace containing the particular microbe is 
introduced into fresh fermentable substance, this latter undergoes 
the same fermentation ; further, it is shown that, however great the 
number of accidental non-specific bacteria which may be introduced 
at the same time, unless that particular bacterium be present 
amongst them, the specific fermentative change does not ensue. 
The same is the case with infectious diseases : the number of non- 
specific bacteria in water, dust, air, various common articles of food, 
<tc., is sometimes great, but no amount of these would set up any of 
the infectious diseases, like cholera or typhoid fever, tetanus or 
diphtheria ; in order to do so there must be amongst them the 
particular microbe of cholera or typhoid fever, &c. Again in each 
fermentative process the substance which is to undergo the fermen- 
tation must be susceptible of the particular fermentation : a sub- 
stance that contains sugar can undergo the alcoholic fermentation, 
a substance that contains alcohol can undergo the acetic acid fer- 
mentation, &c. The same is the case in the infectious diseases : an 
individual must be susceptible to the disease, though it is not quite 
clearly established what the meaning of this is. Further, just as 
in the fermentative process the susceptibility of the substance alone 
is not sufficient, is only a preliminary condition, the actual infection 
with the specific microbe being the essential, so also in the infectious 
disease : in order that a susceptible individual should become the 
subject of the disease, it is essential that the specific microbe should 
be present and should find entrance into this susceptible individual. 
Just as little as a particular condition of the atmosphere, of tempe- 
rature, &c., is capable of producing the souring of milk, so also a 
particular atmospheric or telluric condition, season, or other external 
circumstances alone cannot produce an infectious disease. What is 
wanted in the first place is the presence of the Bacterium lactis in 
the one, the specific pathogenic microbe in the other ; atmospheric 
or telluric conditions may and do favour the more rapid multiplica- 
tion and dissemination of the Bacterium lactis or other specific 
microbes, but without the presence of the specific microbes these 
processes could not take place. “ Thunder in the air ” could not 
turn the milk sour, could not make meat tainted, could not turn 
beer or wine sour, without the presence of the specific microbes, 
which by their presence and multiplication produce those undesired 
changes in these substances ; the particular condition of the air 
could and would increase their rate of multiplication and distribu- 
tion, and therefore increase the chances of infection of these sub- 
stances, and consequently a more conspicuous manifestation of the 
effects of the activity of those microbes, but it could not produce the 
microbes themselves. 
