THE PURITY OF DRINKING-WATERS 139 
But this is not true of the types likely to be found in sewage. Sew- 
age contains every form of human excretions; and since it is by 
means of the excretions that the pathogenic bacteria find exit 
from the diseased patient, it will thus be easily understood that 
sewage-contaminated water is likely to contain bacteria which are 
pathogenic for man. Such water is therefore always dangerous, 
a fact abundantly proved by the great prevalence of water-borne 
diseases in cities whose water-supply is contaminated with sewage. 
When the bacteria in water are in the thousands per c.c., the water 
is unsafe, not because this number of bacteria is injurious, but 
because such water is commonly sewage-contaminated. 
When we recognize the great chance which sewage-contami- 
nated water has of becoming impregnated with the germs of human 
diseases, it is a little surprising to learn that the number of kinds of 
disease actually distributed by water is very small. Only one of 
our common diseases is known to be frequently distributed by 
water. This is typhoid fever, in regard to which the evidence is 
abundant and conclusive. This evidence need not be given here, 
but it is sufficient to demonstrate that typhoid fever is very com- 
monly acquired from drinking-water, that the danger comes wholly 
from water which has in some way become contaminated with 
human excrement, usually through sewage, and that the drinking 
of sewage-contaminated water is probably the most prolific source 
of this dreaded and serious disease. 
Other water-borne diseases are of less importance. Asiatic 
cholera is distributed by water, but this is, at least in this country, 
of no significance. Certain forms of dysentery are probably dis- 
tributed by water, but little is known of this matter as yet. No 
other diseases are known to be thus distributed. 
Detection of Sewage Contamination.—Sewage contamination 
is a rapidly growing danger. As population increases, the amount 
of sewage also increases, and it becomes more and more difficult 
to dispose of it so as to prevent its contaminating the sources of 
drinking-water. Many a stream formerly used for drinking pur- 
