112 
BACTERIOLOGY. 
lie dormant for years and later on, when the resistance 
is lowered by disease or by unsanitary conditions of 
living, become active and cause tuberculosis in what¬ 
ever organ or tissue they may be lodged. The milk 
from cattle having tuberculosis of the udder is the 
most dangerous but even when the udder is healthy and 
the disease located in other organs, the milk may con¬ 
tain tubercle bacilli. Not only is the milk from tuber¬ 
culous cattle infectious, but also the products—butter 
and cheese—made from the milk. From what has been 
said, it is easy to see the danger of using raw cows’ 
milk for infant feeding without positive assurance that 
the cows have been tuberculin tested and are free from 
tuberculosis. 
The Bacteria in Water. 
Water as it falls in the form of rain is free from 
bacteria. It begins to be contaminated with bacteria 
when it reaches the dust-laden air above the earth, and 
after it reaches the ground the number of bacteria is 
greatly increased from the soil. As it drains from the 
surface of the earth or percolates through it, it is 
classed either as surface water, of which ponds, lakes, 
or rivers are examples, or as ground water, which 
feeds wells. Surface water always contains large 
numbers of bacteria, but the water in wells contains 
only a few on account of the filtering action of the 
soil. While the number of bacteria in surface water 
is large, there is going on constantly processes of puri¬ 
fication which keep the number in check. 
