is
handled, whether on the farm or elsewhere should be screened.

It should be kept in mind in the handling of milk and other dairy
products that human food is being prepared and that cleanliness is
desirable from every point of view, and that the methods of
handling and production should compare with those used in the
preparation of foods which like milk cannot be cleaned when once
polluted. Desirability, keeping quality, healthfulness and the value
of every product made from milk depends upon the extent and amount
of contamination.

CHAPTER IV.

INFECTION OF MILK WITH PATHOGENIC BACTERIA.

That the disease-producing, or pathogenic bacteria, are able to
infect milk supplies is shown by the fact that numerous epidemics of
contagious disease have been directly traced to milk infection. Milk
is generally consumed in a raw state, and as a considerable number
of this class of organisms are able not only to live but actually
grow in milk, which is such an ideal culture-medium for the
development of most bacteria, it is not surprising that disease
processes should be traced to this source. The organisms in milk
capable of causing disease do not alter or change its physical
properties sufficiently to enable their presence to be detected by a
physical examination.

=Origin of pathogenic bacteria in milk.= Disease-producing bacteria
may be grouped, with reference to their relation toward milk, into
two classes, depending upon the manner in which infection occurs:

Class I. Disease-producing bacteria capable of being transmitted
directly from a diseased animal to man through the medium of
infected milk.

Class II. Bacteria pathogenic for man but not for cattle, which are
capable of thriving in milk after it is drawn from the animal.

In the first group, the disease produced by the specific organism
must be common to both cattle and man. The organism must live a
parasitic life in the animal, developing in the udder, and so infect
the udder. It may, of course, happen that diseases toward which
domestic animals alone are susceptible may be spread from one animal
to another in this way without affecting human beings.

In the second group the bacterial species live a saprophytic
existence, growing in milk, as in any other nutrient medium, if it
happens to find its way therein. In such cases, milk indirectly
serves as an agent in the dissemination of disease, by giving
conditions favorable to the growth of the dis