128 BACTERIA IN WATER. 
from a score or more per c.c. in very pure waters to a few hundreds 
in a moderately pure reservoir; and from this number to many 
thousands in streams which are badly contaminated with sewage. 
I. THE PURITY OF DRINKING-WATERS. 
In determining the purity of drinking-water we are not so much 
concerned with the number of bacteria it contains as with the kinds. 
Very large numbers may be present and yet the water may be 
perfectly wholesome, while, on the other hand, with only a small 
number present the water may be deadly. As we shall see in a 
later chapter, the bacteria in milk may be reckoned by the millions 
per c.c., and yet the milk may be perfectly healthful; and at the same 
time bacteriologists regard with suspicion water that contains them 
only in thousands. The reason for this difference is simple. When 
milk contains these large numbers they are almost sure to be harmless 
types; but if water contains even a few thousands, the typhoid 
bacillus is likely to be among them. It is impossible to condemn 
any sample of water simply from the number of bacteria which 
it contains; still the number serves as a useful measure of purity 
for the following reason: Water that is fairly pure and contains 
only the bacteria liable to come from ordinary sources seldom contains 
more than a few hundreds of bacteria per c.c.; it is only water 
that is receiving contamination from sewage or some other source 
of decaying filth that contains large numbers of bacteria. Hence, 
the finding of large numbers of bacteria in a water-supply suggests 
sewage contamination and the water at once becomes suspicious. 
SEWAGE CONTAMINATION. 
Water may receive hosts of bacteria from various sources, but 
the one great and almost only source of real danger is from sewage 
contamination. Most of the types of bacteria found in nature 
and in natural water are perfectly harmless, so that it makes little 
difference whether they are abundant or few in the water we drink. 
But this is not true of the types likely to be found in sewage. Sew- 
