PARASITIC ORGANISMS. 19 
Bacteria grow and reproduce in Pasteur’s solution, render- 
ing it opaque, as well as in almost all fluids that abound in 
proteid matter. That such fluids readily putrefy is owing to 
the presence of bacteria, the vital action of which suffices to 
break asunder complex chemical compounds and produce new 
ones. Some of the bacteria require oxygen, as Bacillus an- 
thracis, while others do not, as the organism of putrefaction, 
Bactervum termo. 
Bacteria are not'so sensitive to slight variations in tempera- 
ture as most other organisms. They can, many of them, with- 
stand freezing and high temperatures. All bacteria and all 
germs of bacteria are killed by boiling water, though the spores 
are much more resistant than the mature organisms themselves. 
Some spores can resist a dry heat of 140° C. 
The spores, like Torula and Protococcus, bear drying, with- 
out loss of vitality, for considerable periods. 
That different groups of bacteria have a somewhat different 
life-history is evident from the fact that the presence of one 
checks the other in the same fluid, and that successive swarms 
of different kinds may flourish where others have ceased to 
live. 
That these organisms are enemies of the constituent cells of 
the tissues of the highest mammals has now been abundantly 
demonstrated. That they interfere with the normal working 
of the organism in a great variety of ways is also clear; and 
certain it is that the harm they do leads to aberration in cell- 
life, however that may be manifested. They rob the tissues of 
their nutriment and oxygen, and poison them by the products 
of the decompositions they produce. But apart from this, their 
very presence as foreign agents must hamper and derange the 
delicate mechanism of cell-life. 
These organisms seem to people the air, land, and waters 
with invisible hosts far more numerous than the forms of life 
we behold. Fortunately, they are not all dangerous to the 
higher forms of mammalian life; but that a large proportion 
of the diseases which afflict both man and the domestic animals 
are directly caused, in the sense of being invariably associated 
with, the presence of such forms of life, is now beyond doubt. 
The facts stated above explain why that should be so; why 
certain maladies should be infectious; how the germs of dis- 
ease may be transported to a friend wrapped up in the folds of 
a letter. 
Disease thus caused, it must not be forgotten, is an illustra- 
