yphoid
bacilli, present in countless numbers in the feces of persons suffering
or convalescent from typhoid fever, find their way into streams, lakes,
and wells.[91] They retain their vitality, and when they enter the
digestive tract of an individual, rapidly increase in numbers. Numerous
disastrous outbreaks of typhoid fever have been traced to contamination
of water. Coupled with the sanitary improvement of a city's water
supply, there is diminution of typhoid fever cases, and a noticeable
lowering of the death rate. Many cities and villages are dependent for
their water upon rivers and lakes into which surface drainage finds its
way, with all contaminating substances. Mechanical sedimentation and
filtration greatly improve waters of this class, but do not necessarily
render them entirely pure. Compounds of iron and aluminium are sometimes
added in small amounts, under chemical supervision, to such waters to
precipitate the organic impurities. Spring waters are not entirely above
suspicion, as oftentimes the soil through which they flow is highly
polluted. All water of doubtful purity should be boiled, and there are
but few natural waters of undoubted purity. There is no such thing as
absolutely pure water in a state of nature. The mountain streams perhaps
approach nearest to it where there are no humans to pollute the banks;
but then there are always the beasts and birds, and they, too, are
subject to disease. There are very few waters that at some time of the
year and under some conditions are not contaminated with
disease-producing organisms. No matter how carefully guarded are the
banks of lakes furnishing the water supply of cities, more or less
objectionable matter will get in. In seasons of heavy rains, large
amounts of surface water enter the lakes, carrying along the filth
gathered from many acres of land drained by the streams entering the
lakes. Some of the most serious outbreaks of typhoid fever have come
from temporary contamination of ordinarily fairly good drinking water.
In general, too little attention is given to the purity of drinking
water. It is just as important that water should be boiled as that food
should be cooked. One of the objects of cooking is to destroy the
injurious bacteria, and they are frequently more numerous in the
drinking water than in the food.

The argument is sometimes advanced that the mineral matter present in
water is needed for the construction of the bone and other tissu