SYMPOSIUM ON PUBLIC HEALTH PROBLEMS 93 
factory than the use of hypochlorites, and with present war 
prices for chemicals, it is considerably cheaper than treatment 
with hypochlorites. 
The use of cheap sterilizing agents has not, as was at one 
time feared, displaced filtration as a means of water purifica- 
tion for several reasons. First, the public having been edu- 
cated to the superiority and ready availability by means of 
filtration of a clear and limpid water, now as a rule demands 
that such water be furnished, and in fact, muddiness has been 
as great a friend to water supply improvement as has the fear 
of disease. In the second place, sterilization with chlorine 
and hypochlorites has its limitations in that it usually imparts 
a taste and odor to the water to which it is applied, especially 
when large quantities must be used to counteract the effect 
of heavy pollution, much organic matter, and high turbidity. 
It is therefore becoming the custom in connection with all 
public water supplies where turbidity and color must be re- 
moved, to place the main reliance upon filtration, and to use a 
sterilizing agent as an auxiliary or finishing treatment, and as 
an additional factor of safety. 
The real test of the effect of purified water supplies on the 
public health is to be found in its influence upon communicable 
diseases that may be water-borne. The most important of 
these diseases in the United States at the present time is ty- 
phoid fever. Because of the relative hardiness of the organism 
of typhoid, this disease has not been so easy to eliminate from 
the country as cholera. 
In a survey of vital statistics relating to typhoid fever in 
the United States, Johnson has shown that in 1900 the typhoid 
fever death rate in the registration cities of the United States 
was 36 per 100,000. In 1915, the typhoid fever death rate in 
registration cities was 15. Thus there has been a reduction 
of 58 per cent. This remarkable reduction is believed to be 
due entirely to improvements in public water supplies, because 
these cities that have always had a pure water supply show 
only a slight decrease in the typhoid fever death rate. 
We see better the real effect of the improvement in the 
public water supplies by considering individual cities. In 
Philadelphia the typhoid death rate before the installation of 
