198 
PRINCIPLES OF BACTERIOLOGY 
or rather, if they are not carried there by dead organic 
matter and excrement. For full detail of these organ¬ 
isms, see special chapter given to each. 
Microorganisms of the Water 
Nothing is more important to a community than that 
it is supplied with pure water. Not only that it is clear 
and palatable, but it must be free from pathogenic bacilli. 
The eye is no criterion in this matter, for water that 
may look clear and pure will often contain many harm¬ 
ful bacteria, while cloudiness and odors may be due 
to recent disturbances, to harmless plants, etc. 
Most waters contain many bacteria, but the presence 
of pathogenic bacteria is due to pollution either by sewer¬ 
age or imperfect drainage. Where the water supply is 
obtained from a well, great care must be taken in select¬ 
ing a site that is free from the drainage of stables, privy 
vaults, etc. In the city where the source of supply is 
from lakes and rivers, constant watch must be kept to 
see that the sewage and industrial wastes do not render 
it unfit for use. Pools and swamps are often a menace 
to public health, for while this water may not be taken 
into the body for drink, they furnish a breeding place 
for insects that often carry disease germs. 
A large body of water tends to be self-purifying. 
When contamination occurs by the natural processes, i. 
e., sedimentation, oxidation, and the continual action of 
sunlight, these harmful bodies are destroyed. Sedi¬ 
mentation is the gravitating of the particles to the bot¬ 
tom, carrying with them impurities and bacteria. The 
action of oxygen and sun rays destroys many bacteria, 
especially those on the surface. 
Water-borne Disease. —When pollution occurs, the dis¬ 
ease germs that are usually found are the typhoid-colon 
