bacteria are disease producing. Such information can be secured only by
microscopic examination, and only then by persons who have a knowledge
of such matters.

29. Since, as has been pointed out, bacteria cling to all dirt, the
dirt that milk contains is one of the causes of souring and putrefaction
of milk, and may be a cause of disease. Indeed, it is definitely known
that dirty milk sours much more quickly than does clean milk. Actual
tests in which clean milk was put in a cool place have proved that it
will keep for weeks, whereas dirty milk will sour in a day or two,
especially in warm weather. This information should point out clearly to
the housewife that it is not merely heat that changes milk or causes it
to sour. She should understand in addition, that bacteria grow and
multiply very rapidly when conditions for their growth are provided.
These conditions are moisture, warmth, and the right kind of food, and
as all of these are found in milk, this product is really ideal for
bacterial development. The only way in which to protect milk is to make
sure that no bacteria enter it, or, if they do, to make it impossible
for them to grow. This may be done by keeping the milk so cold that they
cannot thrive, or by destroying them in various ways, which are taken
up later.

30. In former times, there was not much danger of wide-spread disease
from the milk supply, for it was cared for almost entirely by those who
kept a few cows and distributed milk to a small number of customers. In
fact, it has been only within the past 50 years that large quantities of
milk are handled by separate dairies and shipped great distances from
the source of supply and that the distribution of milk has become a
great industry. When so much milk is handled in one place, it is more or
less unsafe unless the dairy is kept extremely clean and is conducted in