99 
Taking the B. coli content as an indication, it is evident that the 
filtered water during the 1907 and 1908 periods was of a better 
quality than it was in 1906. 
In our report for 1907 ® we stated that — 
The improvement in the water being followed by the lower rate of prevalence of 
typhoid fever in the summer of 1907 suggests cause and effect. * * * However, 
it should be borne in mind that the prevalence of typhoid fever in Washington and 
other communities has varied considerably in different years during which the con- 
ditions as to water, etc., have, so far as known, remained approximately the same. 
The good results following the purification of the drinking water 
sometimes appear rather gradually. In some cities the full effects 
have not been apparent until two or three years following the filtra- 
tion of the water supply. Washington has been furnished filtered 
water since October, 1905. In the first year after filtration (1906) 
there was no lowering of the typhoid death rate coincident with the 
improvement in the water supply. In fact, the rate was slightly 
higher than in 1905. In the next two years (1907 and 1908) there 
was a considerable decrease in the prevalence of the disease during 
the warm weather season (Jul}^, August, and September). The rates 
for the other (cool) seasons of these three years, however, were almost 
^ exactly the same. 
The lower prevalence of typhoid fever for 1907 and 1908 was coin- 
cident with a still greater improvement in the water than was attained 
in 1906 
Concerning the relation of the Potomac River water to the preva- 
lence of typhoid fever in W ashington, there is still some difference of 
opinion. Three main questions now present themselves: 
1. Was much of the typhoid fever in the District of Columbia 
caused by infection in the water supplied by the Potomac River 
before the filtration of the water? 
2. Was much of the typhoid fever in the District of Columbia in 
. 1906, the first year after filtration, caused by water-borne infection? 
3. Was some of the typhoid fever in the District of Columbia in 
1907 and in 1908, the second and third years after filtration, caused 
by infection introduced by the filtered Potomac River water? 
The evidence is not yet sufficiently complete to give definite and 
final answers to the first two of these questions. While much may 
be said on both sides of these questions, it will be necessary to await 
the results of still further observations on the typhoid-fever situation 
and of further bacteriologic studies of the water before arguments 
sufficiently complete to point to fairly definite conclusions can be 
presented. 
Hygienic Laboratory Bulletin No. 44, p. 44. 
